J. Trowbridge — A Stcmdard of Light. 129 



long and one-twentieth of an inch wide, connected with a lever 

 by which its expansion might be measured, would yield at 

 2000° a light suitable for most purposes."* 



It has been urged against this standard that different speci- 

 mens of platinum will emit different amounts of light with the 

 same difference of potential; and that it would be difficult to 

 carry out a measurement of the light and the strength of the 

 current all at the same instant. With a view to obtaining a 

 knowledge of the practical difficulties in this measurement, I 

 interposed a fine platinum wire between the poles of a battery, 

 and endeavored to measure the light emitted, together with the 

 difference of potential at the extremities of the wire and the 

 amount of current which passed through a tangent galvano- 

 meter. The difficulties, however, in using a fine platinum 

 wire with a moderate battery power were great. The wire 

 would fuse before the measurements could be satisfactorily 

 made. I then employed a strip of platinum foil 5 mm wide, 

 about 5 cm long, and about -02 mm in thickness. This was placed 

 in a shunt circuit of a small Gramme machine in order that if 

 the strip should fuse the dynamo machine might not race. 

 With the proper speed and a suitable adjustment of resistances, 

 the light from this platinum strip could be maintained very 

 constant. 



The strip was placed in a long Ritchie photometer box, 

 which was provided with two mirrors inclined according to the 

 plan of Ritchie. One half of the photometer disk was illum- 

 inated by the incandescent strip, and the other half by a sperm 

 candle. 



The electrical current was measured by a tangent galvano- 

 meter of which the reduction factor was "44 in the C. G. S. 

 system. The difference of potential at the ends of the strip was 

 measured by a Thomson quadrant electrometer, the deflections 

 of which were compared with that of a Daniell cell, the electro- 

 motive force of which was approximately 1*09. A Thomson 

 voltmeter was also used. The indications of this instrument 

 agreed with those of the electrometer. The following table 

 gives the deflections of the instrument: 



Deflection of Tangent Galva- Deflection of the Electro- 



nometer in Degrees. meter in Centimeters. 



63 5-3 Light the color of a candle. 



61 4-9 



59 4-6 



57 4-3 



54-5 3.8 



53-75 3-4 Light very dull red. 



One Daniell cell gave a deflection with the electrometer of 



* Scientific Memoirs, p. 45. 



