. Radio-activity . ion per cubic centimetre per second. The great advantage of an apparatus of this kind lies in thefact that the current measured is due to the ionization inside thevessel and is not influenced by the ionization of the external air orby electrostatic disturbances. Such an apparatus is very convenientfor investigating the very penetrating radiations from the radio-elements, since these rays pass readily through the walls of theelectroscope. When the electroscope is placed on a lead plate 3 or4 mms. thick, the ionization in the electroscope, due to a radio-active body placed und
. Radio-activity . ion per cubic centimetre per second. The great advantage of an apparatus of this kind lies in thefact that the current measured is due to the ionization inside thevessel and is not influenced by the ionization of the external air orby electrostatic disturbances. Such an apparatus is very convenientfor investigating the very penetrating radiations from the radio-elements, since these rays pass readily through the walls of theelectroscope. When the electroscope is placed on a lead plate 3 or4 mms. thick, the ionization in the electroscope, due to a radio-active body placed under the lead, is due entirely to the verypenetrating rays, since the other two types of rays are completelyabsorbed in the lead plate. 53. A modified form of electroscope, which promises to be ofgreat utility for measuring currents even more minute than thoseto be observed with the type of instrument already described, hasrecently been devised by C. T. R Wilson1. The construction of theapparatus is shown in Fig. Fig. Proc. Camb. Phil. Soc. Vol. 12, Part ir. 1903. 74 METHODS OF MEASUREMENT [CH. The case consists of a rectangular brass box 4 cms. x 4 3 cms. A narrow gold-leaf L is attached to a rod R passingthrough a clean sulphur cork. Opposite the gold-leaf is fixed aninsulated brass plate P, placed about 1 mm. from the wall of thebox. The movement of the gold-leaf is observed through twosmall windows by means of a microscope provided with a micrometerscale. The plate P is maintained at a constant potential (generallyabout 200 volts). The electrometer case is placed in an inclinedposition as shown in the figure, the angle of inclination and thepotential of the plate being adjusted to give the desired sensitive-ness. The gold-leaf is initially connected to the case, and themicroscope adjusted so that the gold-leaf is seen in the centre ofthe scale. For a given potential of the plate, the sensitivenessdepends on the angle of tilt of the case. There is a certain cri
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