Heat produced by Emission of Ions from Hot Bodies. 633 



a galvanometer deflexion whose direction is independent o£ 

 that of C . 



If the distribution of the thermionic current is quite 

 symmetrical about the centre of resistance of the filament 

 the bridge galvanometer will be exactly compensated, both 

 in the " filament shunted " and the " standard shunted " 

 arrangements. The compensation here referred to is that 

 which is necessitated by the direct disturbing action of the 

 thermionic current on the Wheatstone's bridge circuit. The 

 theory of the compensation has been fully discussed in our 

 previous papers * on the converse heating effect. The 

 present method of experimenting affords no means of test- 

 ing whether the compensation is precise or not ; so that it is 

 necessary to be sure that the method used eliminates any 

 effect arising from lack of balance. In all except the first 

 three experiments below the thermionic current was satu- 

 rated, so that its distribution about the filament must have 

 been determined solely by the distribution of temperature 

 about the latter. With a given distribution of thermionic 

 current from the surface of the filament the distribution of 

 the equivalent current in the filament and in the Wheatstone's 

 bridge circuit is entirely determined by the distribution of the 

 various resistances. Thus the magnitude and direction of the 

 galvanometer deflexion due to lack of compensation will be 

 independent of the direction of O , except in so far as the 

 direction of C may affect the distribution of temperature 

 and resulting thermionic emission about the filament. As- 

 suming that the thermionic current doubles in every 30° 

 rise of temperature, and that the absolute temperature of 

 the filament varies as the square root of the energy supplied 

 locally, an effect might arise in this way which would make 

 the observed effects about 2 per cent, too big with the largest 

 thermionic currents used. With the smaller thermionic 

 currents this effect would be quite inappreciable. The data 

 available for this estimation are not very precise, but they 

 have been selected in doubtful cases so as to lead to an upper 

 limit, i. e. so as to over-estimate rather than under-estimate 

 the effect. Neglecting this effect for the present, the de- 

 flexion due to incomplete compensation will be determined 

 by the thermionic current T and the distribution of the 

 resistances in the circuit, and will be independent of the 

 direction of C . Let the part of the galvanometer deflexion 

 which is due to this cause be D^pyfr, where p is the galvano- 

 meter deflexion due to unit change of the resistance R. 

 yjr is thus the value of the galvanometer deflexion due to 



* Loc. cit. 



