99 
1879.] L. Schwendler— On Electric Light Measurements. 
suring the speed continuously, and is an invention of Lieut, G. S. Clarke, 
B. E., and Professor McLeod. 
The principle of the cycloscope consists in employing a tuning fork 
or reed as the standard by which to measure velocities of rotation. The 
instrument is described in the Proceedings of the Eoyal Society, 1877, Yol. 
XXVI. 
Resistance measurements. See Appendix VI. 
An ordinary Wheatstone bridge with S.U. has been employed. 
By careful comparison I find, 
1 B.A.U. = 1 0509 S.U. 
The resistance measurements are all correct within 0 - 01 S.U. 
Standards of Light employed. See Appendix VII. 
The Standard Candle. 
The Metropolitan Gas Act of .1800 defines the Standard Candle as 
follows :— 
“ Sperm candles of six to the pound, each burning 120 grains an hour. 
To keep the flame of a candle at exactly the same position in the 
photometer, a condition required for accurate photometric measurements, 
I qilaee the candle in a closely fitting tube against the top rim of which a 
spring pn’esses the burning candle. 
The Platinum Light Standard ( P.L.S.) 
The unit of light is defined, as the light emitted from a pnece of pmre 
platinum weighing (x'j grammes and having the most convenient shape and 
size, when a constant current of (y) webers passes through it. 
The current can be easily kept constant by a convenient battery, and 
by inserting in the circuit a galvanoscopie and adjustable mercuiy-iheostat. 
The deflecting wire of the galvanoscope should consist of one thick coppier 
wire only. On the galvanoscope the point is marked off which eorrespiouds 
to a current of (y) webers. 
This Standard piroduces a light of perfect constancy, so long as the 
current is kept constant, and further allows a correction to be made if the 
■variation of the current is known. 
It is further a standard which can be reproduced very accurately 
anywhere, and its magnitude may be altered to any extent to suit certain 
practical purposes, by simply varying the elements of the weight, shape 
and size of the pilatinum, or the strength of the current passing through 
it. 
It does not alter in itself. Hence the Platinum Light Standard fulfils 
all the conditions of a good standard, and I therefore propose it should 
be used in future as the standard of light in England, in lieu of the 
standard candle. 
