12 METHODS OF MEASUREMENT [CH. 
air in the cylinder is to be accurately measured, it is advisable to 
enclose the supporting and charging rod and sulphur bead inside a 
small metal cylinder M connected to earth, so that only the charged 
gold-leaf system is exposed in the main volume of the air. 
In an apparatus of this kind the small leakage over the sulphur 
bead can be almost completely eliminated by keeping the rod P 
charged to the average potential of the gold-leaf system during 
the observation. This method has been used with great success by 
C.'T. R. Wilson (loc. crt.). Such refinements, however, are generally 
unnecessary, except in investigations of the natural ionization of 
gases at low pressures, when the conduction leak over the sulphur 
bead is comparable with the discharge due to the ionized gas. 
52. The electric capacity C of a gold-leaf system about 4 cms. 
long is usually about 1 electrostatic unit. If V is the decrease of 
potential of the gold-leaf system in volts in the time ¢ seconds, the 
current 2 through the gas is given by 
pote 
ae 
With a well cleaned brass electroscope of volume 1 litre, the 
fall of potential due to the natural ionization of the air was found 
to be about 6 volts per hour. Since the capacity of the gold-leaf 
system was about 1 electrostatic unit 
‘= ee = 56x10 Es. units = 1:9 x 10-” amperes. 
With special precautions a rate of discharge of 1/10 or even 
1/100 of this amount can be accurately measured. 
The number of ions produced in the gas can be calculated if 
the charge on an ion is known. J.J. Thomson has shown that the 
charge e on an ion is equal to 34x 10-™ electrostatic units or 
113 x 10~* coulombs. 
Let g=number of ions produced per second per cubic centi- 
metre throughout the volume of the electroscope, 
S = volume of electroscope in cubic centimetres. 
If the ionization is uniform, the saturation current 7 is given by 
t= qe. 
