160 



H. F. Reid — Theory of the Bolometer. 



v-w 



(/R) 



Art. XIV. — Theory of the Bolometer; by Harry F. 

 Reid, Ph.D. • 



The bolometer consists of a Wheatstone's bridge in which 

 the resistances are first adjusted so that no current passes 

 through the galvanometer. Two arms of the bridge are made 

 of thin platinum strips ; when one of these is exposed to the 

 radiation of a hot body it grows hotter, increases in resistance, 

 thus destroying the balance of the bridge and producing a 

 deflection of the galvanometer needle, which measures the in- 

 tensity of the radiation.* 



i Let ti, v, w, be currents 



and' R, m, S, IS, G, B, be 

 resistances in the Wheat- 

 stone's bridge, as in the dia- 

 gram (fig. 1) ; and let E be 

 the E. M. F. of the battery. 

 By Kirchhpff s law for the 

 distribution of currents we 

 find 



w = AT > 



where J is a function of the 

 resistances in the various 

 parts of the bridge. 



The ordinary thermoelec- 

 tric forces at the various junctions balance each other and do 

 not affect the currents in the bridge. The thermoelectric 

 forces due to the Peltier effect are very small and are quite 

 negligible ; when the four arms of the bridge are equal, their 

 effect is merely to change slightly the total E. M. F. ; we shall 

 see later that this is unimportant. 



We can replace E by its equivalent in terms of the current 

 passing through the arm &R, which contains the exposable 

 bolometer strip. (For convenience we shall designate the vari- 

 ous arms of the bridge by their resistances). This current is 

 essentially equal to the current v in the branch R when the 

 bridge is balanced, since w is always very small in comparison 

 to v.\ We have 



v=E GS(l+l)+Sl(R + S) 

 A ' 



vR(k-l) 



' ' G(l+0+*(K + S) 



* For a detailed account of this instrument, see Proc. Am. Acad., 1881 ; this 

 Journ. Ill, vol. xxi. p. 187, March, 1881. 



f Professor Langley " On Hitherto Unrecognized Wave-lengths ;" this Journ., 

 Ill, vol. sxxii. Aug., 1886, foot note p. 94. 



