Heat produced by Emission of Ions from Hot Bodies. 625 



positive ions thus liberated bombard the hot wire and 

 produce a heating effect which vitiates the measurements. 

 At the lowest temperatures the measured effect was from five 

 to ten times as large as the calculated. These discrepancies 

 led Wehnelt and Jentzsch to the conclusion that the theory 

 was insufficient completely to account for the phenomena 

 which accompany the emission of electrons from hot 

 conductors. 



This may be so ; but it seems to us possible that the 

 source of the discrepancy is to be found in the method of 

 experimenting. None of the investigators referred to have 

 taken precautions to eliminate or allow for the direct dis- 

 turbing action of the thermionic current on the galvano- 

 meter of the Wheatstone's bridge system, which is referred 

 to in our papers* on the heat liberated during the absorption 

 of electrons by metals. It is impossible to estimate the 

 magnitude of this effect from the data given in the papers, 

 but our own experience leads us to judge that it may well 

 be much greater than the effect under investigation. We 

 urge the possibility of this explanation with a certain amount 

 of diffidence as we have not made experiments on lime- 

 coated wires ourselves, and it is barely possible that there is 

 something about the emission from lime that makes its 



CD 



behaviour exceptional. 



All the experiments that we have made up to the present 

 have been carried out with filaments of osmium as the source 

 of emission. We are glad to be able to take this opportunity 

 of thanking the Deutsche Gasgliihlicht Actiengesellschaft of 

 Berlin for presenting us with the material. The cooling- 

 effect was obtained by measuring the change of resistance 

 of a heated filament which occurred when the thermionic 

 current was turned on and off. The filament was heated by 

 a direct current which was constant except for the distur- 

 bance produced by the thermionic current. The thermionic 

 current was controlled by a suitable potential difference 

 applied to a surrounding cylindrical electrode. It was started 

 and stopped by reversing this potential difference. The 

 change of resistance was compared with that produced by a 

 known change in the main heating current, which thus 

 supplied a known increment of energy to the system per 

 unit time. If this increase in the rate of supply of energy 

 is expressed in watts, the cooling effect per unit thermionic 

 current may be obtained directly in equivalent volts by a 

 comparison of the observed deflexions of the galvanometer 

 of the Wheatstone's bridge. 



* Phil. Mag. vol. xx. p. 173 (1910) ; vol. xxi. p. 404 (1911). 

 Phil. Mag. S. 6. Vol. 25. No. 148. April 1913. 2 U 



