Iv | NATURE OF THE RADIATIONS 103 
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 1s greatly diminished by placing over the active material a 
metal screen which absorbs the a rays, but allows the 6 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. 24) is connected with an electrometer 
by the wire 7. The disc and wire are completely surrounded by 
insulating matter 7. 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 mm a depression in 
a lead plate AA. 
: C77 Electrometer 

Fig. 24. 
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 a 
1 C. R. 130, p. 647, 1900. 
* The activity of the radium preparation was not stated in the paper. 
