628 Profs. Cooke and Richardson on Absorption of 



the whole process was repeated with the 2000 ohm resistance 

 connected across the filament instead o£ across the standard 

 resistance CE. The new readings yield an independent 

 value of <E>. 



The galvanometer Gr was o£ the D'Arsonval type supplied 

 by Nalder Brothers, London. It had a resistance o£ 225 

 ohms and gave 1 scale-division for 1*29 x 10~ 8 ampere. It 

 was very heavily damped. The filaments used were 15 mm. 

 long and *065 mm. in diameter. To saturate the thermionic 

 current a potential difference of from 40 to 100 volts was 

 applied, the amount necessary varying in the different 

 experiments. 



It will be noticed that the method of measurement em- 

 ployed is equivalent to using the hot filament as a bolometer. 

 The change in the resistance of the filament will be propor- 

 tional to the change in the rate at which energy is supplied 

 to it, provided the change in the energy supplied is small. 

 The details will be clearer after we have considered the 

 magnitude of some of the effects to be observed. 



llieory of the Measurements. 



Let us first consider the method of calibration. This 

 depends upon the measurement of the steady deflexion De of 

 the galvanometer G which occurs when the resistance M is 

 shunted by the resistance N". Let the resistance of M be z 

 and that of N y. If the resistance of the filament when K" 

 is not shunted is R and the current flowing through it i, the 

 rate of supply of energy to the filament is 



E = Ri 2 . 



When M is shunted by N the current i becomes i + "di, and 

 consequently R is changed to R + ^R. Thus the new steady 

 rate of heat-supply is 



E + dE = (R + BR)u + d0 2 - 



To the first order in the small quantities dR and ~di, which 

 is all that is required, the increment in the rate of energy- 

 supply is 



3E = 2Ridi + e 2 3R (2) 



To determine "di in terms of the quantities measured we 

 proceed as follows. Let V be the electromotive force of the 

 battery Bj and R the resistance of the whole circuit in- 

 cluding the Wheatstone's bridge. Let V = R / l5 then i x is 

 the current supplied by the battery Bj. The current i 

 which heats the filament is smaller than i h since a small 



