1879.] L. Schwendler—On a new Standard of Light. 85 
effect on a given point. These difficulties might be overcome to a certain 
extent by mechanical means ; as, for instance, by cutting the wick automa- 
tically within equal and short intervals of time, and by placing the candle 
in a closely fitting metal-tube, against the top rim of which a spring presses 
the burning candle, in fact a similar construction to that used for carriage 
candles. But to say the least, all such arrangements are cumbersome. With- 
out going into further details with reference to the Carcel Burner, it may be 
said that the disadvantages of this standard are at least equally great. In 
fact it appeared to me that the production of a standard light by combus- 
tion is not the right method ; the flame resembles too much organic life 
with its complex and incessantly varying nature. Gauging mechanical 
force by the power a particular horse of a certain breed is able to exert, 
can scarcely be called a less scientific standard, than the combustion stan- 
dard for measuring light. Under these circumstances, I thought it best 
to leave the old track, and produce the standard of light, by the heating 
effect a constant current has, in passing through a conductor of given mass 
and dimensions.* 
New Standard of Light.—Several Platinum Photometric Standards 
were made and tried. If the current passing through the platinum was 
kept constant, the light produced was also constant, and for the same cur- 
rent and the same platinum standard, the light was always of the same 
intensity, under whatever other circumstances the experiments were con- 
ducted, 
Platinum evidently is the best metal which can be chosen, for it does 
not change in contact with oxygen ; it can be procured very pure and its 
melting point is high enough to allow an intense light. 
It is probable that at a high temperature platinum becomes volatilized, 
but this process can only be exceedingly slow, and therefore the light pro- 
duced by a standard, cannot alter perceptibly in time. To make the light 
constant from the moment the current passes, 7. e., to establish dynamic 
* The idea of using the light produced by a conductor through which a strong 
current passes, as the unit of light, appearedto me so natural and simple, that I 
_ could scarcely understand why it had not been proposed and acted upon before. 
. I could however find nothing on the subject anywhere, until lately my attention 
; was called toa small pamphlet written by Zollner in 1859 in which the same idea 
occurs : 
In the preface to his Inaugural Dissertation, Zéllner says :— 
; “ andererseits aber auch zu zeigen, dass ein galvanisch glihender Platindraht von 
“den bis jetzt bekannten Lichtquellen zur Aufstellung ciner photometrischen Hin- 
“heit, trotz mancher practischer Schwierigkeiten, vielleicht dennoch das gecignetste 
«4 Mittel sei.” 
I have since learnt that pr.“Draper, as early as 1844, proposed a “unit lamp” 
consisting of a platinum strip heated by an electric current. 
1l 
