536 Prof. J. J. Thomson on the Charge of Electricity 



normally. M. Langevin has shown that this abnormal ioni- 

 zation is practically absent when the surfaces are wet. 



The coil and focus-bulb producing the rays were placed 

 in a large iron tank elevated on supports; in the bottom of 

 the tank a hole was cut and closed by an aluminium window. 

 The vessel A was placed underneath this window and the 

 bulb giving out the rays some distance behind it, so that the 

 beam of rays escaping from the tank were not very divergent. 

 The rays were reduced in intensity to any required degree 

 by inserting different numbers of layers of tinfoil or sheets 

 of aluminium between the bulb and the vessel. The tank 

 and the aluminium plate at the top of A were connected with 

 earth and with one pair of quadrants of an electrometer. The 

 other pair of quadrants were connected with the water-surface 

 B; this surface was charged up by connecting it with one of 

 the poles of a battery consisting generally of two Leclanche 

 cells, the other pole of which was connected with earth. 

 After the surface was charged it was disconnected from the 

 battery and the insulation of the apparatus tested by observing 

 whether there was any leak when the Rontgen rays were not 

 on : ihe insulation having been proved to be good, the rays 

 were turned on, when the charge began to leak; by measuring 

 the rate of leak, the quantity of electricity crossing in one 

 second the gas exposed to the rays can be determined if the 

 capacity of the system is known. The effective capacity of 

 the system consisting of the discharging vessel, the connecting 

 wires, and the quadrants of the electrometer depends to a large 

 extent on the charge in the electrometer, and increases so 

 quickly with the charge that the rate of movement of the 

 spot of light reflected from the mirror of the electrometer 

 increases but slowly when the charge in the electrometer is 

 increased beyond a certain value. The reason for this is 

 shown by the following investigation. 



Let Q x be the charge on the system consisting of the pair 

 of quadrants and the apparatus connected with it, V x the 

 potential of this pair of quadrants, V 2 the potential of the 

 other pair, and V 3 the potential of the needle ; then we have 



Ql = 211^1 + ^12^2 + ^13^3, 



where q n , gi 2j <?i3 are coefficients of capacity. Let 6 be the 

 azimuth of the needle, then if the two pairs of quadrants and 

 the needle are at the same potential, Qx will not depend upon 

 6 if the quadrants are symmetrical with respect to the axis of 

 the needle. Hence 



d$ + dd ■*" d& 



