Heat produced by Emission of Ions from Hot Bodies. 627 



across a high resistance slide-rheostat Rh 2 , and various 

 potential differences could be tapped off and read by the 

 voltmeter V. The switch K 2 enabled this potential to be 

 instantaneously reversed. From K 2 the electrons flowed to 

 the middle point of a 2000 ohm resistance. In the standard 

 shunted arrangement the ends PQ of this were connected to 

 C and E respectively. This is equivalent electrically to 

 making the thermionic current start from the middle o£ the 

 arm OE of the bridge. In the "filament shunted" arrange- 

 ment the ends PQ of the 2000 ohm resistance were connected 

 respectively to E and F. This is equivalent electrically to 

 a current startino- from the centre of resistance of the filament 

 and returning to its surface in the same proportion in various 

 parts as the densities of the currents of (negative) electrons 

 away from it bear to one another. If the emission of 

 electrons is symmetrical about the centre of resistance, the 

 thermionic current will produce no direct disturbance of the 

 galvanometer in either of these cases. In the second case 

 this will be true in any event, but in the first only provided 

 the resistances are adjusted for no deflexion. The thermionic 

 currents were measured by the microamperemeter T. 



The method of carrying out a set of observations was as 

 follows. The bridge current having been turned on, Ph 2 

 was adjusted until the thermionic current was of the desired 

 value. After conditions had become steady the resistance in 

 the arm DF was altered until the bridge was balanced, with 

 the thermionic current stopped by opposing the potential 

 supplied by the battery B 2 . A series of half-minute readings 

 of the galvanometer G and the microammeter T were then 

 taken with the thermionic current alternately on and off. 

 Previous experiments had shown that half a minute allowed 

 ample time for the new steady condition to be established 

 and for effects due to the period of the galvanometer to have 

 disappeared. Readings in opposite directions were taken in 

 order to eliminate the steady drift due to fluctuations in the 

 temperature of the filament arising from other causes. The 

 mean of the successive deflexions was taken to represent the 

 effect sought. Then, with the thermionic current off another 

 set of half-minute readings on the galvanometer were taken, 

 the key K 3 being now alternately closed and open. The 

 mean of these gives the effect due to a known change in the 

 energy supplied to the filament. The voltage of the battery 

 B x and the reading of the ammeter A were then recorded. 

 This constitutes one set of readings. A similar set was then 

 taken with the key K x reversed. From these two sets, as 

 we shall see, the value of <E> may be deduced. As a rule 



2U2 



