86 L. Schwendler—On a new Standard of Light. [No. 2, 
equilibrium between the heat produced and the heat lost per unit of time, 
it is necessary to make the arrangement in such a manner, that the electric 
resistance offered by the standard is only in the piece of platinum, intended 
to be made hot by the current, and not in the other parts of the circuit. 
For this reason I find it best to cut the piece of platinum out of a 
platinum sheet. 
Figure 1, Plate VIII gives the form in actual size. The two ears, left 
white in the drawing, may then conveniently form the electrodes between the 
leading wires and the piece of U-shaped platinum which has to produce the 
light. As the U-shaped portion is left in its natural connection with the 
ears, the contact takes place over a large surface, and therefore the contact 
resistance must be small. This special form, if the dimensions are defined as 
well as the weight of the platinum sheet, out of which it is cut, can be easily 
reproduced anywhere. Further it is required to exclude the draught from 
the heated platinum. This is best done by putting on a cover of thin white 
glass. One half of it is left white, the other half is blackened on the inside. 
This precaution is required in order to insure that light omanniang. from 
one side only of the platinum is used in the photometer. 
Otherwise light from the back part of the heated platinum, would be 
reflected into the photometer. This part is unknown and could there- 
fore not be taken into account when measuring the light emanating from 
one side of another light. In fact to be able to form right conclusions 
from Photometric measurements, it is necessary to arrange the experiment 
in such a manner that either the two lights under comparison throw 
the same fraction of the total light into the Photometer, or if this is 
impossible, to ascertain this proportion accurately. ; 
The Platinum Standard light (PSL), described before, we will call in 
future A. Sending acurrent of 6.15 webers through it (15° deflection on 
my large Tangent Galvanometer, for which the constant = 2.296 C. G. S.), 
the PSL (A) produces a light equal to 0.69 Sugg’s candle, or, 
1 Sugg’s candle = 1.44 PSL (A) with 
6.15 webers. 
Hence, if this particular light were adopted as the unit, we might 
define it as follows :— 
6.15 webers passing through a piece of Platinum 2 mm. broad, 
36.28 mm. long and 0,017 mm. thick, weighing 0.0264 Gm., having a cal- 
culated resistance = 0.109 S. U., and a measured resistance = 0.143 8. U. 
at 66° F. gives the unit for Hehe intensity.* 
* Tn order to show that a platinum light standard can easily be reproduced, I will 
give here some actual measurements :— 
The Platinum sheet out of which the P, 8. L. (A) was cut weighed 0.0364 Gm, 
per square centimetre, From this the weight of the part which becomes hot calculated, 
