TH E 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES. ] 



Art. XXX.— The Bolometer ; by S. P. Langley. 



In the number of this Journal for March, 1881, there 

 appeared an article descriptive of the actinic balance (since 

 called the Bolometer), an instrument which has gained 

 acceptance among physicists as a useful aid in the study of 

 radiant heat. It was, it may be remembered, originally devised 

 by the writer to discriminate the heat in any small portion of 

 the grating spectrum, but it has since found wider applica- 

 tions. 



As at first constructed, the strips, representing arms of the 

 Wheatstone bridge, were made of iron from 0*001 to 0-0001 of 

 an inch in thickness. The instrument was even under these 

 initial conditions, very many times as sensitive as the best ther- 

 mopile the writer then possessed, but there does not appear to 

 be any definite statement as to the exact sensitiveness in its 

 early form. 



In the article referred to, however, the instrument is repre- 

 sented as giving a deflection of about 40 scale divisions (milli- 

 meters) from the lunar heat, concentrated by a thirteen-inch 

 lens, and it was sufficiently accurate to give a probable error of 

 rather less than one per cent for a single observation on a con- 

 stant source of heat, so that the accuracy of the bolometer 

 (quite a distinct consideration from its sensitiveness) was even 

 then as great as that of the best photometric process. The 

 galvanometer in use at that time was one of the early Thomson 

 pattern made by Elliott. 



The first bolometers were made by the writer's own hands. 

 Subsequently the strips were usually cut out from sheets of 

 thin platinum, and in one or two instances made from flatted 

 wire, the strip of the linear bolometer at that time (about 1883) 



Am. Jour. Sci. — Fourth Series, Yol. Y, No. 28. — April, 1898. 

 16 



