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
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
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.
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
With suitable precautions an electroscope can thus readily measure an ionization current corresponding to the production of 1 ion per cubic centimetre per second.
The great advantage of an apparatus of this kind lies in the fact that the current measured is due to the ionization inside the vessel and is not influenced by the ionization of the external air or by electrostatic disturbances4. Such an apparatus is very convenient for investigating the very penetrating radiations from the radio-elements, since these rays pass readily through the walls of the electroscope. When the electroscope is placed on a lead plate 3 or 4 mms. thick, the ionization in the electroscope, due to a radio-active body placed under the lead, is due entirely to the very penetrating rays, since the other two types of rays are completely absorbed in the lead plate. If a circular opening is cut in the base of the electroscope and covered with thin aluminium of sufficient thickness to absorb the α rays, measurements of the intensity of the β rays from an active substance placed under it, can be made with ease and certainty.
58. A modified form of electroscope, which promises to be of great utility for measuring currents even more minute than those to be observed with the type of instrument already described, has recently been devised by C. T. R. Wilson5. The construction of the apparatus is shown in Fig. 13.
The case consists of a rectangular brass box 4 cms. × 4 cms. × 3 cms. A narrow gold-leaf L is attached to a rod R passing through a clean sulphur cork. Opposite the gold-leaf is fixed an insulated brass plate P, placed about 1 mm. from the wall of the box. The movement of the gold-leaf is observed through two small windows by means of a microscope provided with a micrometer scale. The plate P is maintained at a constant potential (generally about 200 volts). The electrometer case is placed in an inclined position as shown in the figure, the angle of inclination and the potential of the plate being adjusted to give the desired sensitiveness. The gold-leaf is initially connected to the case, and the microscope adjusted so that the gold-leaf is seen in the centre of the scale. For a given potential of the plate, the sensitiveness depends on the angle of tilt of the case. There is a certain critical inclination below which the gold-leaf is unstable. The most sensitive position lies just above the critical angle. In a particular experiment Wilson found that with an angle of tilt of 30° and with