The Three Secretaries 217 



He, therefore, invented a new instrument, which he called 

 the bolometer, a thermometer of almost infinite tenuity and 

 delicacy, which measured minute degrees of radiant heat with 

 an accuracy unknown to the thermopile and greater than that 

 of any photometric process, and which at the same time pos- 

 sesses a sensitiveness to radiant energy only less than that of 

 the eye, being able in its recent constructions to recognize 

 variations of this energy corresponding to not over one- 

 millionth part of a degree on an ordinary thermometer. 



This instrument was made in part at the cost of the Ameri- 

 can Academy of Arts and Sciences, as administrators of the 

 bequest of Count Rumford, and its completion was announced 

 in the paper sent to the Society December 8, 1880, and read 

 at its meeting of January 12, 1881. The years 1879 and 

 1880 were devoted to elaborating and perfecting it. 1 



The action of the bolometer is based upon variation of 

 electrical resistance produced by changes of temperature in a 

 metallic conductor, such as a minute strip of platinum. This 

 strip forms one arm of an electric balance, and the change in 

 the strength of the electric current passing through it, be- 

 cause of this change of resistance, is registered by a delicate 

 galvanometer. Its sensitiveness is greater than that of the 

 most delicate thermopile possible, and its accuracy of meas- 

 urement has a corresponding advantage. One of the earliest 

 results of the bolometer work was the demonstration experi- 

 mentally that the maximum of heat in the normal spec- 

 trum is in the orange, and not, as was formerly supposed, 

 in the infra-red portion ; but a larger field opened for it in 

 the exploration of the infra-red portion, whose existence 

 was first suspected by the elder Herschel. The bolometer 

 showed that this region contained three-quarters of the solar 

 energy. Before the invention of the bolometer the distribu- 



1 "The Bolometer and Radiant Energy." Proceedings of the American Academy of 

 arts and sciences, 1880-81 ; Volume xvi, pages 342 to 358. 



15 



