504 Mr. W. B. Haines on Ionic Mobilities 



experimental chamber. Two parallel circular brass plates 

 of 10 cm. diameter are separated by a glass cylinder which 

 is fitted with taps lor the inlet and outlet of the gas. The 

 central part of the upper plate is of copper gauze, giving 

 communication between the main chamber and a small side 

 chamber. In this latter the ions are generated by the a-rays 

 from a deposit of polonium on the centre of a small piece of 

 copper situated at A. By maintaining a small potential 

 difference between the brass disk B and the upper plate, the 

 ions of the sign required can be drawn to the gauze, where 

 they diffuse into the main chamber and come under the 

 influence of an alternating field applied between the plates. 

 The use of sealing-wax for assembling the parts and of 

 ebonite for insulation are indicated in the drawing. Fig. 2 



shows the simple electrical connexions, omitting keys. The 

 central part of the lower plate is insulated from the rest, and 

 is connected to a carefully-screened Dolezalek electrometer, 

 so serving to measure the current carried by the ions. An 

 induction-coil supplied the alternating voltage between the 

 plates, the primary circuit containing an alternating machine 

 and an adjustable lamp-resistance. The voltage between the 

 plates was indicated by a Kelvin multicellular voltmeter. 



If d be the distance between the plates, and the voltage 

 applied be of the form a sin pt, then the ions travel a distance 

 2uajpd from the upper plate before the field reverses ; u 

 being the velocity in unit field. Then a current will begin 

 to pass between the plates when 



ua 



pd 



