The rate of movement of the gold-leaf is observed by a reading microscope through two holes in the cylinder, covered with thin mica. In cases where the natural ionization due to the enclosed air in the cylinder is to be measured accurately, 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 eliminated almost completely 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. cit._). 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. =57.= 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 t seconds, the current i through the gas is given by _CV_ _i_ = ---- _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. If the ionization be uniform, the saturation current _i_ is given by _i_ = _qSe_. Now for an electroscope with a volume of 1000 c.c., _i_ was equal to about 1·9 × 10⁻¹⁵ amperes. Substituting the values given above _q_ = 17 ions per cubic centimetre per second. With suitable precautions an electroscope can thus readily measure an ionization current corresponding to the production of 1 ion per cubic centimetre per second.