182 J. Trowbridge — A Standard of Light. 



other was exposed to the radiation of a given surface of plati- 

 num. The faces of the thermopile were provided with the 

 customary cones, and a series of diaphragms of thick card-board 

 extended between the radiating surface of the vessel containing 

 the heated water and the platinum strip. The thermopile was 

 connected with a short coil galvanometer, and was moved until 

 the galvanometer needle came to zero. This arrangement was 

 extremely sensitive — a movement of a centimeter in the posi- 

 tion of the faces of the pile being sufficient to drive the spot of 

 light from the galvanometer mirror off the scale, corresponding 

 to a movement of nearly fifty centimeter scale divisions. 

 There is no difficulty in effecting a balance as quickly as an 

 ordinary photometric measurement is made. While one ob- 

 server compares a candle or other source of light with the light 

 from an incandescent strip of platinum, another could make 

 the measurements with the thermopile, and could obtain the 

 amount of energy radiated by the incandescent strip in terms 

 of the constant source of heat. It is necessary to reverse the 

 faces of the thermopile, or to place a second constant source of 

 heat on the same side upon which the incandescent strip is 

 placed. The following table indicates the character of the 

 results : 



Deflection of 



the Tangent 



Galvanometer. 



Tempera- 

 ture of 

 Water. 



Distance of 

 Face of Pile 

 from Water. 



Distance of 

 Face of Pile 

 from Strip. 



Remarks 





o 



° C. 



cm. 



cm. 









57-5 



95 



26-5 



49-5 



Dull 



red. 





56'5 



95 



28-0 



48 









61 



96 



25-5 



• 50-5 



Bright yellow 



62-5 



96-5 



24-5 



5T5 



a 





a 



62-5 



96 



23-0 



53 



cc 





cc 



60-5 



97 



25-7 



50-3 



it 





a 



58"5 



97-5 



26-7 



49-3 



cc 





a 



60 



95 



24"2 



51-8 



a 





a 



62-2 



94 



23'7 



52-3 



<C 





cc 



The reduction factor of the galvanometer was 44 in the 

 C. G. S. system. When the photometric indications were the 

 same, the thermopile indicated a large change in the amount 

 of heat received. Thus the heat indications within the range 

 in which the experiments were taken were far more sensitive 

 than the photometric indications. 



It seems possible, therefore, to assume as a standard of light 

 an incandescent strip which radiates a definite amount of 

 energy, this energy being measured at a fixed distance which 

 will best agree numerically with the absolute system of meas- 

 ures now universally adopted in heat and electricity. The 

 method of Draper and Schwendler could be combined with the 

 methods I have described above. For a practical standard, a 



