94) L. Schwendler—On a new Standard of Light. [No. 2, 
I regret that I have not been able to calculate from the above results 
the temperature of the heated platinum, since I could not procure in time 
a copy of Dr. William Siemen’s Bakerian Lecture (1871), which at present, 
to my knowledge, is the only source where the increase of resistance of 
platinum at high temperatures ean be found. 
To sum up :— 
The advantages of the new standard of light are :—The light is 
perfectly constant if the current be kept constant ; it allows a eorrection 
to be made for the variation of the current if this variation is known ; 
it can be reproduced accurately everywhere if ordinary precautions be 
taken to secure pure platinum ;* its magnitude can be altered to any 
extent to suit certain practical purposes by simply varying the elements. 
of weight, shape, and size of the platinum, or the strength of the current 
passing through it; it does not alter of itself either in intensity, size, 
or position, and therefore by it most accurate photometric measurements 
can be executed ; the standard can be easily made to fit into any adopted 
system of absolute units, Hence the new standard fulfils all the recog- 
nised conditions of a perfect and rational standard, and therefore it would 
be advisable to adopt it in future, as the practical standard for light- 
measurement. There would be no practical difficulties met with in the 
introduction of the new standard for technical purposes. 
* The conductivity of any metal is much lowered by slight impurities, and plati- 
num does not form an exception. Hence great care must be exercised in the selection 
of platinum for the light standard. Dr William Siemens in his Bakerian lecture 
says: “The abnormal resistance of some platinum is due chiefly to the admixture of 
iridium or other metals of the same group, and it appears that platinum prepared 
by the old welding process is purer and therefore better suited for electrical purposes. 
than the metal consolidated by fusion in a Deville furnace,’* 
— eS 
