when an Electron escapes from Surface of a Hot Body. 205 



case, the thermionic potentials are placed on one end of the 

 filament. The superposition of the bridge current causes 

 an unsymmetrical emission of electrons, so that the ther- 

 mionic current is not quite symmetrically distributed about 

 the bridge network. In the second case, the superposition 

 of the bridge current causes a shift in the centre of emission 

 from the centre of resistance to which point the current 

 was returned, so that there is a resultant potential difference 

 along the wire. These effects are supposed to reverse with the 

 bridge current. With i first in one direction and then in the 

 other, alternately, high and low values of cj> may be obtained. 

 The mean of these values is supposed to be the correct one. 



In practice these two values are not worked out. The 

 readings for the two values are grouped into a single 

 equation, but the validity of the method depends upon the 

 truth of the above assumption. Inasmuch as Cooke and 

 Richardson did not test the above assumption experimentally, 

 it was deemed advisable to perform such a test in this in- 

 vestigation. The two values of 0, found with i first in one- 

 direction and then in the other, differ by a considerable 

 amount. If, however, an alternating current be used to 

 heat the wire and a small superimposed direct current be 

 used to operate the bridge, two values of <f> should be found 

 that differ by a much smaller amount. This experiment 

 was performed, and the prediction was fulfilled. The two 

 values found for the cooling effect were 4*742 and 4*797,. 

 the mean for the two being 4*766. Unfortunately, an acci- 

 dent prevented the securing of direct-current measurements 

 on this particular filament. However, characteristic values 

 from a similar filament by the direct-current method were 

 6*210 and 3*315, the mean being 4*763. The results of the 

 experiment were regarded as satisfactory proof of the validity 

 of the assumption that the mean of the high and the low 

 values in the direct-current method really eliminated the 

 uncompensated portion of the disturbance produced by the 

 thermionic current in the bridge system. 



The Joule heating effect of the thermionic current presents 

 more difficulty. It cannot be eliminated and can be calcu- 

 lated only approximately. Cooke and Richardson performed 

 this calculation and incorporated the correction term found 

 in the general expression for the total diminution of energy 

 due to the escape of electrons. They found for the total rate 

 of loss the expression 



