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University of California.















October and November, 1910






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Ztt Bttb i^aftiinoii tpttM*

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This Course of Lectures on lUuminatiog EugiDeering was giren at the Johns Hopkins University, Baltimore, between the dates October 26 and November 8, 1910, nnder the joint auspices of the University and the Illuminating Engineering Society. The origin and objects of the lectures are clearly stated in the preliminary announcement of the course, from which the following quotation is made:

" The Illuminating Engineering Society recognizing the fact that there is an increasing demand for trained illuminating engi- neers, and that the present facilities available for the specialized instruction required are inadequate, determined, through an act of the Council of the Society, to encourage the establishment of a course of lectures on the subject of illuminating engineering. This course should have three objects: (1) to indicate the proper coordi- nation of those arts and sciences which constitute illuminating engi- neering; (2) to furnish a condensed outline of study suitable for elaboration into an undergraduate course for introduction into the curricula of undergraduate technical schools; and (3) to give practising engineers an opportunity to obtain a conception of the science of illuminating engineering as a whole.

" Inasmuch as such a course is most appropriately given at a university where graduate instruction is emphasized, and as the Johns Hopkins University has regularly offered courses by non- resident lecturers as part of ita system of instruction, and is now preparing to extend its graduate work into applied sciences and engineering, an arrangement has been effected by which the lectures will be given at this University under the joint auspices of the University and the Illuminating Engineering Society. The sub- jects and scope of the lectures have been proposed by the Society and approved by the University. The lecturers have been invited by the University upon the advice of the Society."

The lectures were attended by 840 men from various parts of the United States, many of them representatives of technical schools, gas and electric central stations, and manufacturing com-



vi Pbefacb

panies. A large nnmbeT of the attendants at the lectures ako followed the course of laboratory work which had been arranged. The general inteieet in the course encourages the hope that these published Tolumes may serve to advance our knowledge of this new and important branch of engineering.

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L Tbm Physical Basib or tkb PsoDU<nios of Lisht. Three

lectureg 1

JoevH B. AuES, Ph. D., Profeeaor of FhrslCB ftnd Direc- tor of the Pbyslcal LHboratory, Tb« Jolins Hopkins Unlvenltr. II. Thb Phtsicai. Chabactkbistios or LcMiNous SoiTBCEs. Two

lecture* 3C

EmVABD P. Htdb, Ph. D., President, Illuminating Bn- glneerlng Sodetr; Director of Pliyaical Laboratorr, National Electric Lamp Assoclatlnn.

HI. Thk Chcuistbt or LTTHntOQa SoiiBciCB. One lecture 93

WiLus R. Whithkt, Pb. D., Director of Reaeardi Labo- rabny. General Electric Co.; Past President, American Chemical Sodetr-

IV. Blectsic Illdkihaktb. Two lecture* 10&

Chauxb P, STxqracBiz, Ph. D., Consulting Bnglneer, General Electric Co.; Professor at Electrical Engineer- Ing, Union University. V. il) Oas aho On. iLLumiTAirra, (3) iNOAKimciifT Gas

MAioTLEa. Tv>o lectures IGT

(1) AixzANi^ C. HumRBETs, M. B^ Hon. Sc. D., Presi- dent of BtaTens Institute of Technology; Past Preaident American Gas Institute.

(S) H. C. WnrrAEBB, B. S., H. 8., Professor of Industrial Cbemlatry, Colombia Unlversltr. VI. Thi Gm^uTioN Am DisTBinrTiON or BLxcTBicrrT with

Special Retebenci v) liiaHTiNa. Two lecture* 331

John B. Whiteheas, Ph. D., Professor of Applied Elec- tricity, The Johns Hoiking UnWenjIty. TIL The UAiruPACTDEE Ain> Dia^msunoH or AKnnciAi. Gas

with Si>BciAL Betebehce to LiGHTiRa. Txoo iectureB 277

(1) Ml E. G. Cowd^t, Tice-President, Peoples Gas Ught and Coke Company, Chicago, Til. <2) Ma. WALTn R. Addickb, Vice-President, Consolidated Gas Co., New York,

Till. PnoTOMnsio Uitits and 8rAin>ABDB. One lecture 887

BnwAiD B. Rosa, Ph. D., Physicist, National Bureau of Standards.

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ii General Contents

IX. Tbb HsASTiBCMEnT or Light. Two lectures 411

Ci^ATTOR H. Shabp, Pb. D., Test OfBcer, Blectrical Test- Ins LftborataiT, New York City; Past PreBident, Illnml- natlng Engineer Ing Society.

X. The AECK1TECT08AL Abpecis or Illuminatino BnoiNBBB-

iBQ. One lecture 607

Walteb Cook, A. M., Vice-President, American Institute of Arcbltects; Past President, Society of Beaux Arts Architects.

XI. Thb Fhtsioumical Asracra or IixirMiNATino BNoinEEBiNO.

Two lecture* 525

P. W. Cobb, B. S., M. D., Physiologist. Physical Labora- tory. National Electric Lamp Association.

XII- The Pstchologioal Asfeots or Iixumihatiko Esoinkeriso.

One lecture 575

RoBEBT M. Tebkes, Ph. D., AselBtant Professor of Psy- chology, Harvard University.

XIII. The Pbinciplgs and Dbbioh of Intebiob Iixuuination. BUe

lectwet 605

(1) W. B. Babbowb, Jb., Assistant Professor Electrical Engineering, Armour Institute of Technology, Chicago. minolB.

(2) L. B. Habkb, B. S., M. M. E., Consulting Engineer, New York Clt^; Past President, Illuminating Engineer- ing Society.

(S) Mb. Nobmait Macbeth, Illnmtnatlng Engineer, The Welsbach Co.

XIV. The Pbircipixs and Desior of DrTraios Illumination.

Three lectures 796

(1) Lome Bell, Ph.D., Consulting Engineer, Boston, Mass.; Past President, lUumtnatlng Engineering Society.

(2) B. N. Wbiohtisoton, A. B., Boston Consolidated' Qas Co.

XV. Shades, Reflectobs and DiTFUsiNa Media. One lecture. . . 885 Van Rensselaxb LanbikoHi B. &., General Manager Holophane Co.

XVI. LioHTiNe FixTUKBs. One lecture 931

Mb. Edwabd F. Caldwell. Senior Member of Firm and Designer, Edward F. Caldwell A Co., New York.

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Geneeal Contents ii

XVII. The CoHiiEBciAi. Abprctb of EIlbctbic LieHTina. One

lecture 946

John W. Lieb, Ja., M. B., Third Vice-President, New York Sdlson Co. ; Past President, American Institute of Electrical Engineers.

XVIII. The Comuebciai. Aspect of Oab Business wrrs Special

Retxbence to Oas Lightino. 0ns lecture 1009

Walton Cubk, M. E., President of The Franklin Instl- tnte, Philadelphia; Third Vice-President, United Oas Improvement Co.. Philadelphia,


Lists of experiments In connection with the Lecture Course, to- together with the necessary bibliographies 1011

Charus O. Bohd, Manager of Photometric Laboratory, United

Gas Improvement Co., Philadelphia. Hebbebt E. Itbs, Ph. D., Physicist, Physical Laboratory,

National Electric Idjnp Association. Pbeston S. MiiXAB, Electrical Testing Laboratory, New York. A. H. PruND, Ph. D., Associate In Pbysles, The Johns Hopkins University.

Index 1047

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By Joseph S. Ames



PhyaUsal Quantitiei and MeaturemMOa Objects and general principles of pbystcs. Uethoda of assigning numbers to iriiyilcal quanUtte>.

a. Uesaurement In tenns of nnlti.

b. Indirect means, e. g., temperature. Simple Ideas.

a. Intnltlve: space and time.

b. Experimental: e. g., force (Illustrated by properties of matter), Units of length, of time, of force; C. O. S.; English.

Derived mechanical quantities, and their units; e, g., density, proMnr&

Heasnrement of length, volume, time, force, pressure.

Errors of Instruments.

Discussion of observations.

Definition of electrical quantities, and their units.

Heasurement of electrical guantltles by portable InstrumeDta.


Energy and Thermal PAenomena

Definition of work and energy; mechanical Illustrations.

Our temp^^tnre sense. Thermal phenomena. Thermal effects.

Methods of producing these effects. Explanation In terms of energy.

Meaning of " Conservation of Energy." Illustrations: battery, dynamo, etc

DiBcnsalon of temperature and Its " measurement."

Discussion of modes of producing beat-effects: flames, friction, conduc- tion, radiation, etc

Radlatlcm and absorption: KlrchhofTs law, " Black Body."

Measurement of energy.

a. Rise In temperature.

b. Mechanical means.

c Electrical method: Bit

* The lectures are based upon the author's text-book " General Physics," published by the American Book Co., New York.

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Illuminatinq Enqineebinq

Radiation Spectra of radiation.

Dlsperelye apparatus.

Detecting and meaBurlng apparatus.

Visible, ultra-violet, tofra-red radiation.

ContlDuous, dlscontlttuouB. and absorption spectra. Modes of producing radiation.

a. " Temperature-radiation."

b. Luminescence: fluorescence, electrical discharge, etc. Color sensation.

Canse of color of natural objects.

a. Body absorption.

b. Surface absorption.

c. Exceptional caaes.

Elxtenalon or temperature acaloe b; radiation methods.

LeCTdee I Physical Qvantities and Measurements

Matter. Through out various seuBes, Buch as those of sight aud hearing, we are constantly receiving seneationa which we interpret objectively; i. e., we locate the cauee of a sensstion in a definite jwrtion of Bpace. We picture to ourBelvee the existence there of something which we call "matter"; and to a limited portion of space which contains matter we give the name " physical body." Matter may be divided into two great claeses: that which is living, such as plants and animals, and that which is not, such as pieces of wood and glass, water and air. Physics is, broadly speaking, ' the science concerned with this second division of matter, which may be called " ordinary matter " ; and phenomena occurring in con- nection with matter of this kind are called "physical phenomeiia,"

The scientific study of a subject involves three distinct ideas; the discovery, the investigation, and the explanation of phenomena. The first two require no discussion here; but it may be well to state that by the words " to explain a phenomenon " is meant to determine its exact connection with other phenomena, to describe it in terms of simpler ones, and in this manner to reduce the number of fundamental ideas as far as possible.

In seeking for explanations of phenomena we assume either directly or indirectly, that there is a definite connection between consecutive events, of such a nature that if we are able to reproduce exactly a definite condition, the same effect will follow regardless

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The Physical Basis op the Productiok of Light 3

of the epoch of time or the location in space. We are justified in this belief by all of our esperience and observationB.

Ether. The careful study of the phenomena of light led philoso- pherE, many years ago, to believe that there is present in space another medium for phenomena than that furnished by ordinary matter. It has become an accepted fact that tliroughout the vast regions of space, in the solar system and beyond, there is a medium permeating all ordinary matter and having many properties in common with matter and yet not- identical with it. This is called " the ether." In order to explain many electrical and magnetic phenomena, and even to describe the phenomena of radiation, it is necessary to assume its existence.*

Vhynai. The object of physics may therefore be defined to be the attempt to determine the exact connection between phenomena, both in ordinary matter and in the ether, and to express these relations with as few hypotheses as possible concerning the nature and properties of either.

Physioal Qnantitiea. A physical quantity is one which we can imagine as capable of changing in amount, something to which we can assign a numerical value. Some quantities can be measured, others cannot. To measure a quantity, another similar one must first be chosen as a standard or i^nit, and then the number of times this is contained in the original quantity is its measure. Thus, a length can be measured in terms of an inch, a yard, a centimeter, etc., depending upon the choice of unit. It is possible to understand the meaning of a zero value of any measurable quan- tity; further, two or more measurable quantities of the same kind, for instance two lengths, may be added. On the other hand there are many physical quantities which cannot be measured; and yet it is possible to give them numerical values. Thus, the temperature of a body cannot be measured, although it is possible by measuring the change in volume of mercury in a thermometer to give a num- ber to temperature.

Simple Quantities. To most physical quantities e^act definitions can be given, but there are a few for which this is impossible; there are no simpler ideas in terms of which we can describe them. The question as to the exact number of these need not be discussed here, and in what follows the philosophy based upon Kant will be

* One Bbould add that a new scbool of philosophy ezleta which looks at nature from a different Btandpolnt

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4 Illithinatinq Enoinebeinq

accepted. According to this we divide our simple ideas into two dBBses; intuitive and experimental. The two intuitive ideas are those concerned with epace and time.

1. A straight line, a polygon, or a solid figure bodnded by plane faces, together with the ideas involved in assigning numerical values to lengths, areas and volumes are conddered intuitive. That is, it is impossible to define what is meant by length ; and the idea of two equal lengths admits of no ambiguity. We can choose a unit length arbitrarily and then, making use of a method of super- position, determine the number to be given any length. The same general metiiod may be applied to areas and volumes.

2. In regard to time, we have a definite conception of what is meant by two equal intervals of time; certain physical phenomena appear to ub to repeat themselves at intervals of time apparently equal, e. g., the vibrations of a pendulum or the balance wheel of a watch. We have no way by which we can prove that these inter- vals are equal, yet there is every reason for believing that these motions of a pendulum and of the balance wheel of a watch are exactly periodic; for at any instant the external conditions affecting the motion are exactly the same, so far as we can tell, as they were at a definite interval of time before. In order to give a number to an instant of time one must choose some periodic motion such as just described, e. g., a certain pendulum vibrating under definite

. conditions, and some arbitrary epoch of time from which to count the number of vibrations; the number of vibrations between the epoch and the instant for which a number is desired is this number. Among the fundamental ideas of which we learn by means of our sens^ may be mentioned temperature, pitch of sound, and what we call " force." For instance, through our muscular sense we become conscious of certain definite sensations when with our hands or arms or bodies we perform certain experiments on matter. Thus, if a large stone is held in the hand we become conscious of a cer- tain property of matter called its "weight"; if we chsnge the motion of a body by means of our arms, e. g., if we throw a ball or stop one in motion, we become conscious through the same chan- nel of a property of matter called " inertia." It is possible, of course, to bold a body suspended from the earth and to set a body in motion or to atop it if moving, by other means than by our muscles; thus a weight can be suspended from a spiral spring and hang at rest with reference to the earth, a compressed spiral ^ring

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The Phtsioal Basis of thb Pboimjotion of Light 6

may, as in & toy gan, produce the acceleration of a bullet, etc. Under all these conditionB which are in their nature identical vith tlioee brou^t about by our mnsdeB -we say, in ordinary language, that "a force ie acting on" the body; but it should be borne in mind that this ie simply a deBcription, nothing more. In order to assign a numerical value to a force one follovs the natural way of studying the simplest cases of forces one can have, and then using definitions and methods based upon these observations. The discussion of this subject forms that branch of mechanics known OS dynamics.

The simplest mode of obtaining a unit or standard force, at least from the standpoint of the inhabitants of this earth, is nndoubt^ edly as follows: 1. Select arbitrarily a certain piece of matter. 2. Suspend it from a fiied support by a cord. 3. Call the tension in tJiiB cord a unit force. It is easy to see how, by means of a pulley, it is possible to balance this force by an equal one obtained by su^ending from the other end of the cord, passing over the pulley, another body which is added to gradually until there is a balance. Having thus obtained two equal forces one can obtain a force twice as great by balancing one body against the two used in the first experiment, etc. In this way a set of standard bodies may be obtained whose weights give forces equal to 1, 2, 3, 4, 5, etc., and then, if it is desired to give a number to an unknown force, this may be done by balancing it against a selection of these known forces.

One can discuss in a similar manner methods of giving numbers to temperature, etc., and this will be done in a later lecture.

TTsiti. The science of mechanics is based upon our ideas of loigth, time and of force, and methods have been discussed showing how we can give numbers to all these quantities. It is seen, how- ever, that in each of these methods certain steps are arbitraiy, and that the number finally obtained depends upon the nature of this arbitrary step.

a. Length. In giving a number to a length the first step is to select a length to which we give the number 1 (if we use the inch, we have one value for the length, if we use the centimeter we have a different value, etc.). The scientific world agrees to adopt as its unit of length the one-hundredth portion of the length of a certain platinum rod, kept in Paris, when this rod is at the temperature of melting ice. The length of this rod under these conditions is

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6 Illduinatiho Gnoineebino

called a " meter " ; and one-hundredth of this length ie called a "centimeter." There are other unit lengths in daily nse in this country and in England, but it ie not neceBsarj to diecuBs them.

b. Time. In assigning a number to an inatant of time we saw that it was necessary to select a " time-keeping mechanism," such Bs a clock, and, secondly, to agree upon some definite instant from which to begin counting. The scientific world has agreed to adopt as its time-keeping instniment the earth itself as it rotates on its axis, and to use as the unit, in terms of which interrals of time are expressed, the " mean solar second." This quantity is the second of time referred to the " mean solar day," which is the average length for one year of the lengths of the solar days during that interval, a solar day being the interval of time between the two instant* when the sun crosses the earth's meridian at any point. It is known that solar days differ in length, but pendulums may be made whose periods are such that they agree exactly with the earth in its rota- tions at intervals a year apart, and these clocks are used ordinarily as time-keeping instruments. Different epochs are chosen in dif- ferent localities; these usually differ by one, two, etc., hours.

c. Force. In assigning a number to a force it was seen that the essential step was to select an arbitrary piece of matter; and here the scientific world has agreed to use a certain piece of platinum kept in Paris. When this body is suspended and allowed to hang vertically there is said to be "a force " in the string equal to the " weight of one kilogram." The thousandth portion of this force is called the weight of " one gram." In England and this country other unit forces are sometimes used, commonly what is called the weight of a " pound."

The unit force on the " centimeter-gram-second " (C. G. S.) sys- tem, as used in all scientific laboratories, is the force required to pro- duce an. acceleration of one centimeter per second per second in a piece of matter whose mass is one gram. This force is called one " dyne." The weight of one gram is very closely 980 dynes it is not the same at all points on the earth.

d. Pressure. From these fundamental properties length, time and force numerous other quantities are derived, one of which should be mentioned here: pressure. By pressure we mean the force per unit area, and, of course, the number we obtain for any pressure depends upon our selection of units of force and of area.

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The Physical Basis op the Pboduction op Light 7

Ueunrementa. It is necesBary to say a few words in regard to the actual meaBurement of, or methoda of assigning numbers to, the phyeical quantities so far discuBsed; but it is easily understood that for any satisfactory discuaeion of the subject reference should be made to some laboratory hand-book.

a. Length. In the measurement of small lengths two methods are in general use; one, depending upon the use of a screw and divided head, the other upon the use of a vernier. In the measure- ment of greater lengths special precaution must be taken against changes due to temperature, flexure, etc.

b. Volume. Measurements of volume are made in one or two ways; if the volume to be measured has the shape of a simple geo- metrical figure, its linear dimensions are measured and its volume calculated ; if the volume is irregular, or if it is that of an inacces- sible space, a method is used depending upon our knowledge of the volume of mercury which is required to produce a definite weight at a definite temperature; e, g., the volume of a bulb may be determined by filling it with mercury, expelling the mercury, noting its temperature, and then weighing it.

c. Time. Methods of accurate measurement of time are too complicated to be discussed here. It is sufGcient to note that there are several metJiods which give an accuracy of a minute fraction of a second,

d. Force. The general method of measuring a force is, as stated before, to balance it against a known force, or a combination of such forces. It is possible to buy sets of weights, or a spiral-spring balance, which will give results sufficiently accurate for all purposes.

e. Pressure. It is ouatomary to measure pressures such as those of the atmosphere, of boilers, of water mains, etc., by balancing the pressure against a vertical column of mercury. An illustration of this method is furnished by the ordinary mercury barometer. Since this is the accepted method, the unit in terms of which pressures are most often expressed is that of " one centimeter of mercury," hy which is meant the vertical pressure required to balance a column of mercury, at the temperature of melting ice, one centimeter in height, when the force of gravity is that which exists at sea-level at latitude 45 degrees. This is a perfectly definite unit, and its value is known in terms of the other units.

Erron of lutminents and Obserratlons. In this brief refer- ence to the measurements of these five quantities it is seen that

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8 Illdhinatinq Enoinebrinq

reliaooe miiBt alwajB be placed upon an inBtniment fumiahed b; some instrument maker; e. g., a micrometer screw, a vernier scale, a Bet of weigbts, a clock, etc., and it ehould not be neceesaiy to empbasize two facts in connection with these instruments. First, every inatmment mnst, of course, be compared with the original standard, or with copies of it whose errors are known. It is for this purpose that in all civilized countrieB Bureaus of Standards exist where such comparisons may be made. Thus every testing laboratory in America has or should have standards of length and of mass, whose values are known accurately in terms of the Paris standards. But, even granting that the testing laboratory has tliese standards, there are many errors or uncertaintieB inherent in the use of every ioBtrument, and a thorough study must be made of it before it can be used for purposes of measurement. Thus no screw has an absolutely uniform pitch, and the variations in this must be determined by known methods; no set of wei^ts is ac- curate, and its errors must be learned; and similar statements are true in regard to every instrument. The first precaution therefore in the measurement of any qnantity is to determine the true scale of the instrument, which is not by any means in all cases that assigned to it by the instrument maker, and also to learn the varia- tions is this scale in different parts of the instrument.

Second, when an instrument is to be used for purposes of meas- urement it is not sufficient to simply make one observation, e. g., to observe onoe the reading on a micrometer of the diameter of a wire. It is necessary to repeat the measurement often. To begin with it is always possible that an error may be made In reading the figures on the instrument or in recording them. Again, when the same measurement is repeated, the measuring instrument being removed and then replaced, it is noted that as a rule a different reading is obtained. This does not mean that the quantity measured has changed or that the instrument used is defective, but simply that in the use of the instrument there are certain inherent errors which limit the accuracy to which it may be trusted, errors coming in part from the individual using the instrument, in part from the instrument iteelf, and in part from other causes. When a sufficient number of observationB have been made one may calculate by known methods the most probable value to be attached to the quantity, and also learn something concerning the certainty with which this number may be regarded as approaching the true value.

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Thb Physicai, Basis op thb Peoduotioh of Light 9

The confidence felt in their meaBuremente by cerUin obBeireTB, «Bd their entire lack of appreciation of the need of ascertaining tiie probable errors and nncertaintiee inTolved, is little short of astoond- ing to one accustomed to ordinary laboratory methods.

Eleotrioal Qaantities. It seems necessary in this, the first lecture of the conrse, to give a brief diseussjon of some quantities which will not be fully explained until later in the coarse. These are the Tarioua electrical quantities ; and, of oourfle, to most engineers they are all well known. In the history of electric currents many unitB have come to the front at different periods, and even at the present time the definitions are not the same in all coontriea. The differ- ences, however, are so slight as to justify ua in neglecting them in all ordinary cases. The definitions glren in what follows are those in terms of which practically all the measuring instruments now in use are calibrated. The unit of resistance the ohm is defined to be equal to the resistance of a column of mercury at zero degrees, of uniform croaa-Bection, of length 106.3 cms., and having the weight of 14.4521 grams. (Thia column then has a crose-section of almost exactly one square millimeter.)

The ampere the unit of current is defined to be auch a current as flowing in a silver voltameter of a specified pattern deposits per second .001118 grams of silver.

The volt the unit of e. m. f. is defined to be such a difference of potential as will produce, when applied to a conductor whose resiatance is one ohm, a current of one ampere.

One of the fundamental properties of current when flowing in a conductor is to develop heat in this condoctor, and it is well known that a simple formula connects the heat developed and the electrical characteristics of the system. This matter will be discussed more fully in the second lecture.

In order to give numbers to the resistance of a conductor the current flowing in it and the difference of potential at any two points, various methods have been devised and instrumente per- fected. At the present time there are no instrumente in common use in laboratories which have attained accuracy to euch a remark- able degree as these. Thia is owing in large part to the epoch- making inrentions of Siemens and Lord Kelvin in Europe, and of Weston in this country. Thanks to the efforts of tiiese scientists we now have instruments for the measurement of volts, amperes and watts which are sufficiently accurate for roost purposes. I may

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be pardoned if I again emphasize the fact, however, that all instru- ments are imperfect and that uncertainty is attached to every ob- servation.

Lecture II

Energy and Thermal Phenomena Work asd Ener^. We are all familiar with the use of the words " work " and " energy " in every-day language. They have been adopted in physics as names of certain physical quantities which admit of exact definition. Naturally these definitions have been made so as to coincide as nearly as possible with those every- day experiences which gave rise to the names originally. Thus, if a man raises a weight vertically from the ground, if he com- presses a spring, if he throws a base-ball, he knows that he is doing work. The essential ideas in all cases of work are, first, the action of a force, and, secondly, a displacement in the direction of this force. Corresponding to these ideas the numerical value of work is defined to be the product of these two quantities, i. e., the value of the force by that of the displacement in the direction of the force. It is easily seen that in all cases in mechanics the results of a force are either to overcome another force or to produce accel- eration (i. e., change of velocity of a piece of matter). Correspond- ing to these two types of fwces there are two ways in which work may be done; first, when a force or opposition is overcome, as when a weight is lifted, a spring is wound up, a bow is bent, etc. ; second, when acceleration is produced, as when a ball is thrown, a fly-wheel or grindstone is set in motion, etc. It is common experience that in all cases when work is done on a body, as when a weight is raised from the earth, a spring is wound, a body given accelera- tion, the body as a result gains the power o( doing work itself. It is said to have gained " energy." If the work done on the body has been done in overcoming an opposing force, the body is said to have gained " potential " energy ; whereas, if the work has been done in producing acceleration, the body is said to have gained " kinetic " energy. Potential energy is therefore always associated with a body in a strained or "unnatural" condition; kinetic en- ergy, with motion, either translation or rotation. It is a matter of common experience also that in all cases of mechanical work one body loses energy and a second body gains it. Thus, if a bullet is expelled from a toy gun by means of the sudden relaxation of a

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The Physical BA6ia of the Prodcction op Light 11

compressed spring, the buUet gains energy and the spring loses it. It is easy to show tiiat for all types of ordinary mechanical forces the amount of energy lost by one part of the system namely, that which is doing work, is numerically equal to the energy gained by another portion of the system, that on which work is being done; and, as a coneequence, therefore, the total amount of energy in the system remains unchanged. It was recognized many years ago that there were certain apparent exceptions which were associated with friction. Thus, if a fly-wheel in motion is disconnected from the driving shaft, its energy as shown by ita motion gradually decreases, as it comes to rest under the action of friction. Here, then, is a case of an apparent disappearance of energy. It was noted, however, that in all cases like this there were certain heat- eSects produced ; and it has been established that there is an inti- mate connection between the loss of mechanical energy and the resulting heat-phenomena. Before stating this connection, how- ever, it may be well to say a few words in regard to our ideas of heat. Heat-Phenomena. Onr attention is called to tiiermal phenomena by means of our temperature sense. We possess in certain portions of the surface of our bodies nerve endings which are sensitive to thermal changes in our environment. That is, if we expose our hands tc sunshine or bring them near a stove in which there is a fire, or to a flame, we experience a definite sensation, and we say that we feel warm. Whereas, if we put our hands on a block of ice, or if we allow some volatile liquid to evaporate from them, we experience a different sensation and say that we feel cold. The first step in the scientific investigation of these phenomena must be taken by exposing a piece of inanimate matter, such as a rod of iron, to the same conditions as those under which we felt warm or cold. When this is done, it is found that the piece of matter undergoes various changes; and these are called thermal effects. In ordinary language we speak of a change from a condition when we feel cold to a condition when we feel hot as being a change from low " temperature " to high temperature. Experiments show that when the temperature of a body is changed, all of its physical properties, with the exception of its mass and weight, are also changed. We select ordinarily from these thermal effects a few of the most obvious and the most important for purposes of study and observation. Among these may be mentioned change in volume, change in electrical resistance, and change in state, as, for instance.



Then a piece of ice melts and becomes liquid. On examination it is found that whenever work is done against friction, heat-effects are produced, and the investigations of Joole led him to believe that the comiection between these two phenomena waa an exact one, which conld be stated by saying that the amount of heat-effect produced depended simply upon the amount of work done against friction, i. e., upon the apparent loss of energy, and upon nothing else, cot upon the time taken for the change, nor the temperature of the working parte, etc. As a matter of fact, if we consider variouB cases in which heat-effects are being produced, we see that in them all work is being done against the smaller parts of the body which experiences the heat-effect, in such a manner that the energy of these smaller parts is altered. Aa a consequence of various experiments, but notably those of Joule, the scientific world has accepted the belief that, when we are dealing with friction or similar phenomena, there is no loss of energy, but that simply the poriiions of matter with which it becomes associated are too minute for observation with our eyes, and therefore ve do not observe by this means the effect produced, but tiiat this effect ia shown to us through our temperature sense or by some heat-effect. This state- ment means that one can apply a numerical value to the heat-effects produced, in such a manner that if it is introduced into the total value of the energy of a system, this total value remains unchanged no matter how much friction may take place in the system.

Conaerration of Energy. This coDstaocy of a certain number when applied to the energy of a system, including in that the proper figure to take into account heat-phenomena, ia an illustration of what is meant by the principle of the conservatiQn of energy. This principle was extended by Joule, Mayer and Helmboltz to include other phenomena than those of mechanics and heat. For instance, we know that, if we place some granules of zinc in a test tube and pour sulphuric acid upon them, there is a violent evolution of gas and the teat tube gets warm. This experiment can be described in terms of energy by saying that the internal energy of the molecules of the zinc and of the acid furnish the supply necessary for the formation of the new molecules and also for the production of the rise in temperature. This experiment forms one of thousands coming under the head of Thermo-Chemistry, and all of these have ' resulted in justifying the above description of the experiment in terms of the internal energy of the various substances. We also

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The Phybioax Babis of the Pkodtjotiok of Lzaht 13

know that, if we take a test tube containing snlphnric acid and insert into it a strip of zinc and a strip of some other metal like copper, the two being joined outside the teet tube by means of some wire, we ^all theu have what we call an electric current. This is an iUastration of a primary cell. In this particular type of cell the zinc diasolvea in the acid, and there la an evolution of gas; the chemical aide of the experiment is exactly the same as in the pieviouB teat-tube experiment just described. It is observed, however, . that in the second ezperimentj that with the primary cell, there is practically no change in temperature of the test tube. This means, in general language, that the energy previously nsed in causing a change in temperature is consumed in this case in producing the electric current As a matt«r of fact, we all know that, when an electric current is passing in a c<mductor, the tem- perature of the latter is raised; and, if the conservation of energy can he extended to the phenomena of electric currents, we would expect to fnd on investigation that the energy consumed in the heating of the conductor by the current is exactly the same as that which is not accounted for in the heating of the test tube where the chemical reactions are going on. Complete investigations on this point justify this belief. Joule performed many interesting experiments to see if in return for a given amount of work he always obtained the same heat-effect regardless of the method and mechanism by which the latter was caused by the former; thus, by meana of a steam engine, it Is possible to turn a paddle in water and one can note the rise in temperature of tiie water, or by meana of the same engine one can turn a dynamo, thus producing a cur- rent which can be made to flow in a wire immersed in water, and again the final effect is the riee in temperature of water. In all cases like this it is found that the conservation of energy is fully justified. As a consequence of these and countless other experi- ments it has become an accepted belief that the conservation of energy can be extended to all phenomena of both matter and ether. Temperatnre and Ihennometcn. Before discussing questicms of radiation and absorption as heat-phenomena it is necessary to say something in regard to temperature and the methods by which we are able to give a number to the temperature of a body. As we use the words hot and cold and speak of high temperature snd low temperature in ordinary language, we are making use of ideas which come from our temperature senaea, and therefore the tem-

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14 Illuminating Engineeeino

perature of a body is a tenn which refers to its relaiive hotness. It is easily seen that this quantity canoot be measured, i. e., we cannot regard otherwise than as absurd such an idea as selecting a unit of hotnese and determining how many times it ia contained in the hotnees to which we wiah to give a number. The words themselTes are nonsense. It is, however, evident that we can choose such a measurable property of some body as changes when the tem- perature of the body changes, and make use of the measured change in this as a means of giving a number to the temperature itself. For instance, we can select arbitrarily a certain cop^r rod, measure its length under some condition which can be easily repeated, such as at the temperature of melting ice,