Page:Radio-activity.djvu/141

 ionization produced by the rays between the plates, any charge given to one of them is almost instantly dissipated. In many cases, the plate does become charged to a definite positive or negative potential depending on the metal, but this is due to the contact difference of potential between the plates, and would be produced whether the rays were charged or not. The ionization of the gas is greatly diminished by placing over the active material a metal screen which absorbs the α rays, but allows the β rays to pass through with little absorption.

The rapid loss of any charge communicated to the top plate can be very much reduced, either by diminishing the pressure of the gas surrounding it or by enclosing the plate with suitable insulators. In their experiments to determine the amount of charge carried by the radium rays, M. and Mme Curie used the second method.

A metal disc MM (Fig. 26) is connected with an electrometer by the wire T. The disc and wire are completely surrounded by insulating matter ii. The whole is surrounded by a metal envelope EEEE connected with earth. On the lower side of the disc, the insulator and the metallic covering are very thin. This side is exposed to the rays of the radium R placed in a depression in a lead plate AA.

Fig. 26.

The rays of the radium pass through the metal cover and insulator with little absorption, but they are completely absorbed by the disc MM. It was observed that the disc received a negative charge which increased uniformly with the time, showing that the rays carry with them a negative charge. The current observed was very small. With an active preparation of radium, forming