OP ARTS AND SCIENCES. 495 



inch long and Tj^yth of an inch wide, connected with a lever by which 

 its expansion might be measured, would yield at 2,000° a light suit- 

 able for most purposes." * 



It has been urged against this standard that different specimens 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 galvanometer. 

 The difficulties, however, in using a fine platinum wire with a mod- 

 erate 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 gram 

 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 illuminated by the incandescent 

 strip, and the other half by a sperm candle. 



The electrical current was measured by a tangent galvanometer of 

 which the reduction factor was .44 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 electromotive force of which was approxi- 

 mately 1.09. A Thomson voltmeter was also used. The indications 

 of this instrument airreed with those of the electrometer. The follow- 

 in<j table gives the deflections of the instrument. 



Light the color of a candle. 



Light very dull red. 



* Scientific Memoirs, p. 45. 



