6 THE THEOBY OF IONIZATION OF GASES 



and falling on the upper plate set free negative ions 

 from the zinc surface. The apparatus was covered with 

 an air-tight glass cover, so that the pressure of the gas 

 could be reduced, and suitable means were provided for 

 turning the micrometer screw and making connection 

 with the plates. The plate B was raised to any required 

 potential by connecting it to a number of small accumu- 

 lators. In a set of experiments in which it was necessary 

 to maintain a uniform force, the number of cells used 

 was proportional to th'e distance between the plates. 

 When the currents passing between the plates are small 

 the conductivities may be measured by a sensitive, 

 quadrant electrometer connected to trie upper plate, 

 but with large currents it is necessary to use an induc- 

 tion balance in order to get accurate determinations. 1 

 The currents must in all cases be comparatively small 

 so that the uniformity of field may not be disturbed by 

 the electrostatic force produced by the separation of the- 

 ions in the gas. 



The following examples may be given of experi- 

 ments with air at 2'5 millimetres pressure. With a 

 force of 350 volts per centimetre, the currents for 

 distances of 1, 8, and 5 millimetres between the plates 

 were proportional to 1, 2'06, and 4'22 respectively. For 

 the same distances the currents were proportional to 

 1, 4*24, and 18'3 when the force was 525 volts per 

 centimetre. The values of a for the different forces 

 may be obtained from the currents. Thus for 350 volts 

 per centimetre the value of a is given by the equation 



2a_2-06_4'22 

 : 1 "2-06* 



1 See paper, Philosophical Magazine, November, 1 903. 



