US888802A - Method of measuring light. - Google Patents

Description

No. 888.802. .PATENTED MAY 26, 19 08;-

' w. J: HAMMER.

METHOD OF MEASURING LIGHT.

APPLICATION FILED OUT. 31.19.05.

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HVVENTOR common forms .the estimates )vith the best substantiall with-int J. nimnsn.

I Pi TliIN-T hli l. C E.

METHOD OF-MEASURING LIGHT.

No. 888,802. Specification of Application filed October 31, 1906.

1 all 101mm if may conccrrz;

Be it known that l, WiLLL-ui J. ll-iiipunma citizen of the United States, residing in New York city, in the State of New York, have invented certain new and useful lmprove ments in ...\lethods of Measuring which the following is a specification. My present invention relates to the art of measuring light, and particularly to an int, )rovement in that art by which lights may be compared without introducing able physiological factor present when estimating ight intensities by the of photometers now in use.

to as comparison. of the lights to be ort-ance, inasmuch as not only do the colors ditlier in the intensituof the effect produced upon the cye of an iinlividual, but there is marked variation between different persons in this respect; so that bv some observers two lights of the same color may be'coln: pared with considerable accuracv, but a change in the: color of the standard will renderthe' determination worthless. i impossibility of maintaina standardlight which will not var Furthermore, the colors compared are of great in from time to timc with the pressure of the specil atmosphere and its with the place of test, and

flame (a amc lie-- mg u'suall employed-for this purpose), offacts the precision of comparison toa greater or less degree. The common'standard of com a-ris'on ls 1 1e now well known Pentane amp; the

colorof this standard is toward the red end practice to have 'a pare a number .of meandescent lamps wl he the selected ary standards, which when burned. at-

give for a. considerable unvarymg light.

Voltage will Obviously,

Light, of;

the varieye, as in the 2 available scientific means of 5 7 ing the changes in the current,

' any investigation 1t is best measuring devices Patented May 26, 1908.

this introduces the lamps will, in spite of all diil'er among themselves, and made with them will necesy thejpersonnl equal ions ol" 1 he different observers. In addition these equations varyin different ways with dill'crent persons, being dependent as already pointed out upon physlo however, l'curacies, since possible care. compz'irisons -sarily dil'l'er b oglcal factors such as the l ocular perception of the individual, varying with fatigue and with physical condition, as 5 is well understood. All of these difficulties 5 the improved method now to be set out; i which consists, briefly, in causing the light to a selected physical and measuring the variations so recision with which the meas- )e effect-ed may he made verv I aim to obviate by 1 urenicnt can great b takmg necessary precautions, mill 1s who] y lndependent of the ccuhurities of I different individuals, or -at .il-slmar be made so by proper care. i will be apparentfrom a consideration of the 1 present state of the art setout above,- I prefer that the quantity to e measured shall be one the variations in which shall not be dependent upon any physiological function for I their accurate perception. Y l Several different ways of embod ing the ;invention are possible. The simp est and 1 best of all those that have occurred to me I 'anging, in an electric circuit a selenium' cell approved construction through which I pass a current, and

Man upon the cell of t test; it being well known that the specific electric resistance of selenium varies wide, In

measuring instrument. I I pose an instrument which will show variation in only one electrical quantity, since in l for well known reasons to employonlyone variable at atime;

ler some con may be used; f my invention I have waves and a y th g -w s;

directly but this, although far more accurate than 1 direct comparisons :oflight by the 'mor dilficult and andwith no compensating a vantage.

For reasons which i he source of currenti is included any desirableform of electrical prefer for this pur.

ditions other forms of more com lex to practice stilllurther inaem to cause eye, c

"40' -wellunderstood.

341,425 filed October 31st, 1906.

In the accompanying drawings Figure 1 is a diagram illustrating the lirs-t of the methods above indicated in my statement of invention, and Figs. 2, S and 4, are modified forms in which sound-wavesareemployed. v In Fig. 1, A isthe battery or other source of current which is to be measured and varia-. tions in which are taken as indicative of light intensities. The batter is-arranged in-cir'-' 'cuit'with a selenium cell B, which is preferably of that form in which the selenium,- after beingcoated upon a suitable conducting support, such as a coilqof nickel wire, is

sealed in a tube of preferably as riearlyperfeet a vacuum as can be obtained, the ends trolling means,' since obvious y such devices ma be employed as the expert may-desire,- an customarily the current will be regulated by the usual resistances and switches in ways Preferablythe current through the cell should-be .of-as low aval'ue,

as can be conveniently maintained constant,v

for scveralreas'ons, one of Which-is that a lar e current evolves so much heat that the eel is subjected to alternate expansions" and 'contractions'which are detrimental; but the most important reason is that for very small currents a proportionately. larger variation of voltage isjcaused vby the action of. the light upon the cell, rendering the device more sensitive and accurate.

'Upon a suitable table I), is arranged the lamp E, to be tested. .At F, I 'have indi rated a, screenhaving a ho le f which may be closed :by a-slide F; in practice it is prefer able to place the selen um cell in alightv ftightlbox M -,(shown partly broken away) having free ventilatiomso that the indications may notgbetafiected by exterior lightg;

and 'inasmuch as the efi'eet to be observed is not detectable by e tjamina'tionofthe cell, it is onlnecessary. to "openthe box at long-in ftervas. v

he method of operating thej"ail'r,ari 'gemeiit guard against change. r

show a second meansof 'cat'ed.

.tion adjacent to the apparatus and lighted, being screened completely from the cell; a

small current, referably a small fraction of an ampere, is t ion caused to How in the cir-' cuit including the cell, and after it has attained a' steady value, the fall of potential around the cell is measured by the volt-meter. After this the light of the standard lamp is allowed to fell upon the cell, and the change in the.

resistance of the circuit (as indicated by the change in the drop) caused by the action of the light upon the selenium is measured and recorded; this then becomes'a constant of the articular cell employed. After this, the lig t which is to be compared with the standard may have itsspecificeffect upon the cell determined in substantially the same way. Obviously the indicationsofthe cellfor thefirst specimens constructed r'nust be calibrated by comparison with the results obtained by 'photometers, since the meas urements of light now in use are purel arbitrary and have no relation to any efinite hysic'a quantity; but this relation having cated may be indefinitely perpetuated by periodic comparisons of different cells to In Figs. 2 and 3, 7 practicing the method'whieh I have indilhis. consists of a siren-disk G of well known construction in which the rows of holes 9 gbear to each other certain definite relations, such asthe notes of'the scale. 01p; posit e to, and rotating with this disk (t e perforated disk being shown detached for clearness of illustration) is a disk H bearing upon-its surface conductors coated with selenium {one for each of, the rows-pf holes g.

driven at constant speed; Inthe circuit with the collector rings and source of current supply isa coil l, disposed so as to afl'ecta tumng-Ifork K provided with a ointer 7c, the" amplit-de of 'v1bration of whic is recorded.

upon the revolving cylinder L, in the manner 0 a-fchronographp The rows of holes in the scale, and may eachbe provided'vvith a simisiremdisk may conveniently be the rela tion to the notes of the octave of a tempered 31.20

lar tuning-fork, which at the normal rate of "rotation will respond to i-ts-ap ropriate tone;

'these being. all substantially he the fork K v and arranged in a similar way, are not illustrated Such ,an' organization will enable t'he same time, when desired] The apparatus thus, described being niaintained' as nearly constant as possible,

125 the observer to'co'nduct a number of tests at ear of the observer; an operator with a good musical car may reach a very close a proximation to a correct comparison by t e relative-intensities of the sounds produced by the two lights; this is, however, open (though in a less degree) to the objections pointed-out with respect to visual photomet'ers.

InFig. 4,-I indicate a construction which is cheaper and et substantially efiective. In this figure theeells B are arranged upon a stationary supportand the siren-disk alone rotates, cutting'oif'and admitting the light by its rotation.

Having now indicated the general outline of the apparatus which I prefer to use and which by anyone familiar with physical manipulations may bealmost indefinitely varied in detail Without departing from. my invention, I will now proceed to describe some of the precautions which I find it desirable to use in order to'effect the measurements towhich I have referred with the greatest facility and precision. Obviously the invention will be employed although the arrangements whichI am now about to indicate should be omitted; but they add so greatly to the precision of the result that in most cases it will be preferable to employ them. r 3

I have already 'mentioned that the selenium cell should be inclosed in a vacuum; the

reason for' this is that the selenium is hy groscopie and when damp there is in such a cell a certain electrolytic action between the selenium and its metallic support which gives rise to an electromotive force in the cell itself, making it to a certain extent a primary battery which may polarize, so that its E. M. F. varies between certain limits, thus Whatever amount of electric effect may be present. In addition the slight heating of the cell causes it to give out gases, changing its internal resistance and'again affecting its E. M. F. These deleterious effects are wholly avoided by inclosing the cell in a highly exhausted receiver; and if in the course of this process a current be passed through the cell suflicient to heat it, at the same, time continuing the action of the pump, it remains constant for a long period of tune'iifI have also found it of advantage to makethe inclosingtube for the selenium cell 6f fused quartz, which has the property of transmitting some rays which are verydargely excluded by glass, and which have a considerable action upon the selenium; this is, so far entirely new with me.

I do not in this a plication, claim tlfiSCOllstruction of the selenium cell itself, which I have found best adapted to my improved I know,

method, since it is described and claimed in adapted to the purpose; Brass and copper wire may also be used; but they are objec- .tionable because a selenite of copper is formed between which and the copper a small E. M. F. may be generated, impairing the accuracy and durability of the cell.

Another precaution which should be em ployed is to standardize the surface of emis-' sion of light so that the same extent of surface of the standard and of the light-source to be measured will be exposed to the selenium cell. One of the best methods of accomplishing' this is to screen the selenium from the light and pass the-portion of light to be measured through a tube the inner surface of which is blackened, of a definite area of cross-section, such as one square millimeter, or in some cases one ;'square centimeter, though iii general the latter unit will be found somewhat large. Obviously, however, whatever unit of surface be chosen should be the same for all of the lights which 'are'to be com ared. Under some circumstances, as w ere the average horizontal candle-power of a rotating lamp is to be taken, the larger surface of exposure may be found most'convenient.

The circuit in which the cell is interpolated should be of very high resistance, not only because this cuts down the current to a very small value, but because it tends to steady it and make it more responsive to the effect to be measured, while less liable to disturbance from other causes. I have used for this purpose resistance as high as 10,000 ohms; and asthe resistances of the selenium cell itself is very' high, the total resistance in the circuit. may be very large indeed. YVith a circuit of these characteristics, I have been by'special apparatus enabled to start and stop large electrical machinery by passing my hand between the cell and the source of light. for somewhat similar reasons the volt-meter or galvanoineter employed should be one of large resistance.

There are other methods of employing measurable physical quantities for the pur poses set out, and all of these which employ Also ' n'amtsul'ing light, which it to be nmmureil phya-li ml qu lmtity,

vr: illtion.

I'JMBELhB'LlTll'IgJ; light, which lruusiiilis-i in mm 'Vttl.lttlfi)l1 i i 2111 amcertaihllhla ply. ntity lTll' otltlfiflmll'by a stmillz'll'll light, mull thlm llltiftizlllll'ill 5 1' thl'i shini- Ila-r variation prmluqml .hy the .li ht to he GOlLllPfitli'flLl. with the stm'lllzlrrl.

3. This new art of imstsillriilg l Mir; cumrellt I (:(Yl'lsists in causing the light to vary elem mvii'ig in u, (Zilfflllill, and meas- 111M110 the extent Oil that variation.

4;. The new art of mhiisuring light, which GUHSlHhS in causing the light 01, :1, stmnlaml lamp to uni the cum n't flow inimi .elvutrim c-itmlit, :"Ll'llll nu ing the extent 0'1 such Vatriathm, than musing at similar variation by 1110mm ml" the light to he Lf/LH'HIMLI'GLl with the;

sliun lnrll, and measuring ita extent;

' .l H witnvq wl'iereof l. have hereunto set my 1: :1, me in 'th presence of two Wi QI'IGSSGE'S.

I WILLIAM J. I'IAINHQIER. l l itnessesg I IRVING M1. UBRLEGHT, ll-Mum BURKE.

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