the Electrical Conduction produced by it. 153 



plate A was spread a uniform layer of uranium oxide. The 

 bottom plate was connected to one pole of a battery of a large 

 number of storage-cells, the other pole of which was to earth. 

 The insulated plate B was connected to one pair of quadrants 

 of an electrometer, the other pair of which was to earth. 

 Under the influence of the uranium the air between the plates 

 A and B is made a partial conductor, and the potential of B 

 tends to become equal to that of A. In order to keep the 

 potential of B at zero, B is connected through a very high 

 resistance T of xylol, one end of which is kept at a steady 

 potential. If the amount of electricity supplied to B through 

 the xylol by the battery is equal and opposite in sign to the 

 quantity passing between A and B, the potential of B will 

 remain steadily at zero. In order to adjust the potential to 

 be applied to one end of the xylol-tube T, a battery was 

 connected through resistance-boxes Rj R 2 , the wire between 

 being connected to earth. The ratio of the E.M.F. e acting 

 on T to the E.M.F. E of the battery is given by 



e 



Ri 



E Bi + R 2 * 



In practice Ri + Rg was always kept constant and equal to 

 10,000 ohms, and, in adjusting the resistance, plugs taken 

 from one box were transferred to the other. The value of e 

 is thus proportional to R 1; and the amount of current supplied 

 to B (assuming xylol obeys Ohm's Law) is proportional to R x . 

 If the resistances are varied till the electrometer remains at 

 the " earth zero," the current between the plates is pro- 

 portional to Ri. If the value of the E.M.F. applied is too 

 great the needle moves in one direction, if too small in the 

 opposite direction. For fairly rapid leaks the current could 

 be determined to an accuracy of 1 per cent.; but for slow 

 electromotive leaks this accuracy is not possible on account 

 of slow changes of the electrometer zero when the quadrants 

 are disconnected. 



The following tables show the results of an experiment with 

 uranium oxide. The surface of the uranium was II cm. 

 square. In order to get rid of stray radiation at the sides 

 lead strips, which nearly reached to the top plate, were placed 

 round the uranium. 16 volts were applied to the resistance- 

 box, and a resistance of 10,000 ohms kept steadily in the 

 circuit. 



