sensitiveness in Detectors of Radiant Heat. 69 



The value of e should be as great as possible. The metals to 

 be used are practically determined by this condition for, on account 

 of the approximate (if not exact) proportionality of the con- 

 ductivities for heat and electricity, per is nearly constant. 



Taking the case of the Bismuth-Antimony couple, we have* 



6=103.10- 6 , ^ = 108. 10- 6 , / o 2 = 41.10- 6 , 



o-! = -0177, <7, = -0442, k = -000252, 

 and hence 



I = 208, A x /A 2 = 2-56, 8 = '4>1\H »JA. 



In practice, however, the length of the pile is far less than 

 two metres, generally being about 2 cms. The sensitiveness may 

 therefore be considerably increased by increasing the length. 

 However, when the length exceeds a few centimetres, the 

 assumption made at the outset, that no heat escapes from the 

 sides of the pile, will not be even approximately true, and so the 

 best length will fall short of that given above. 



Assuming a length of 2 cms., and that A 1 /A 2 =2 - 6, we find 

 that 



8 = 0S\H*JA, G = Resistance of pile x 1091, 



so that even in this case the ordinary rule, to make the gal- 

 vanometer resistance equal to the sum of the other resistances in 

 the circuit, will give a result not appreciably inferior to the best 

 obtainable. The sensitiveness of the pile is about £th of the 

 theoretical maximum. 



5. Bolometer. 



Let the resistance of the bolometer strip at a temperature 

 6 degrees above that of the air be 6(1 +7)6), and let it and the 

 other resistances be arranged as in the diagram. The most con- 

 venient method of solution is not to use the equation deduced by 



bfi + Tjff) 



1896. 



B 

 These and other numbers are taken from the Smithsonian Physical Tables, 



