10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



of the current from a battery would be measured (by the disturbance 

 of the equaHty of the ' bridge ' currents) by means of a galvanometer. 



" This promptness in the action of the metal strip gives it a great 

 advantage over the thermopile for measures of precision. But, beside 

 this, the deflection produced by the single strip and bridge is greater 

 than that from the thermopile, if the element of time enter into the 

 comparison, and still more if the relative areas exposed to radiation be 

 considered. 



"Although (for the reasons just cited) far from as sensitive as we 

 can make it, such a strip then is yet more sensitive than the pile. A 

 number of thermopiles, selected as the most sensitive in the writer's 

 collection, have been exposed to the same source of radiation, placed 

 at the same distance as in the previous experiments. They were 

 .... as follows : 



''A. Large thermopile, by Elliott (Tyndall-lecture pattern), com- 

 posed of sixty-three couples, .... 



" B. Very sensitive thermopile of extra small elements (16 

 couples) .... 



" C. Delicate linear thermopile (7 couples). Working face about 



I mm. by 10 mm. = 10 sq. mm 



" 5". The iron strip, which was about 7 mm. by .176 mm. and whose 



working face was therefore about i sq. mm 



" The time of exposure was about five seconds for the thermopiles 

 and about one-half this for the strip, the latter time corresponding to 

 the rapid swing of the (designedly) insensitive galvanometer. 



" In the table, the first column gives the name of instrument ; the 

 second, the cross-section of the beam of radiant heat which is received 

 upon it ; the third is the actual deflection in galvanometer divisions ; 

 and the fourth the deflection for each square millimetre of exposed 

 surface 



Area Deflection 



Instrument sq. mm. div. Sensitiveness 



A 240 21 1 .9 



B 34 125 3-7 



C 10 147 14.7 



S I 204 204.0 



"After nearly a year's labor (I began these researches systemati- 

 cally in December 1879), I have procured a trustworthy instrument. 

 It aims, as will have been inferred from the preceding remarks, to use 

 the radiant energy, not to develop force directly as in the case of the 



