t.
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
6
i = 1 × ------------ = 5·6 × 10-6 E.S. units = 1·9 × 10-15 amperes.
3600 × 300
With special precautions a rate of discharge of ⅒ or even ¹⁄₁₀₀ of this amount can be measured accurately.
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 3·4 × 10-10 electrostatic units or 1·13 × 10-19 coulombs.
Let q = number of ions produced per second per cubic centimetre
throughout the volume of the electroscope,
S = volume of electroscope in cubic centimetres.