66 Mr Pocklington, On the conditions of 



On the conditions of sensitiveness in Detectors of Radiant 

 Heat. By H. C. Pocklington, M.A., Fellow of St John's 

 College. 



[Received December 19, 1898. Read February 6, 1899.] 



1. The object of this paper is to discuss mathematically the 

 sensitiveness of the two best known detectors of radiant heat, to 

 find what conditions give a maximum of sensitiveness, and, as far 

 as possible, to compare the values of this maximum in the two 

 cases. In each case attention is paid only to the steady state 

 which is reached after the apparatus has been exposed to the 

 radiation for some time. 



2. Nobili's Thermopile. 



Let the thermopile consist of a number of pairs of bars of 

 two metals, joined together alternately and arranged into a rect- 

 angular block with the even junctions on one face and the odd on 

 the other. When radiant heat falls on one face, that face is 

 warmed. The temperature to which it rises must be found by 

 equating the gain to the loss of heat. The heat gained is that 

 fraction of the incident heat which is absorbed. The heat is lost 

 by radiation and convection from the same face, by conduction to 

 the other face whence it is removed by radiation and convection, 

 and on account of the Peltier effect. The heat lost through 

 the sides of the pile is neglected, partly because the sides are 

 generally surrounded by a poor conductor of heat, partly because 

 of the difficulty with which heat is conducted from the inner 

 bars across their insulation to the surface. 



Let heat incident per sq. cm. per sec. = H, 



area of face of pile = A, 



number of pairs of bars = n, 



absolute temperature of air = T, 



rise in temperature of faces of pile = 6 ly 2 , 



difference of thermoelectric powers = e, 



length of each bar = I, 

 areas of cross-section of the separate bars = A 1} A 2 , 



specific resistances of metals = p lt p 2 , 



conductivities for heat = o- l5 cr 2 , 



current = 0, 



resistance of galvanometer = G, 



sensitiveness of galvanometer =\vG, 



