194 Prof. E. Rutherford on the Charge 



the excited activity practically disappears, and the activity, 

 measured by the a. rays, is reduced to one quarter of its 

 equilibrium value. Experiments were made with the radium 

 as nearly as possible at this minimum activity in order to 

 avoid possible complications due to the presence of /3 rays. 

 The latter have almost completely disappeared about three 

 hours after the emanation is removed. The /3 activity is 

 gradually recovered, rising to half its maximum value in 

 about four days. 



A second insulated plate B was placed parallel to the lower 

 plate and a few millimetres distant from it. The plates were 

 insulated in a brass vessel which could be exhausted to a high 

 vacuum by means of a mercury pump. The lower plate A 

 was connected with one pole of a battery, the other pole 

 of which was connected to earth. The current passing 

 between the two plates was measured by a Dolezalek electro- 

 meter with a suitable capacity in parallel. 



If the a rays cany with them a positive charge, this will 

 be communicated to the upper plate in which they are 

 absorbed. At ordinary pressures of the air, however, the- 

 amount of ionization produced by the a particles in their 

 passage through the gas is so large that this charge is rapidly 

 dissipated. It is necessary to work at a very low pressure of 

 the gas, in order to reduce the ionization to a very small 

 value. Under such conditions, it was to be expected that 

 the upper plate would acquire a positive charge. A very 

 different result, however, was observed : on diminishing the 

 pressure of the gas, the current between the plates decreased 

 directly as the pressure, but finally reached a limiting value,, 

 corresponding to about 1/1000 of the value at atmospheric 

 pressure. This current was about the same whether the lower 

 plate was charged positively or negatively, and, although an 

 extremely high vacuum was produced, it was not found 

 possible to further reduce its value. This minimum current 

 was not much altered when hydrogen was present in the vessel 

 instead of air. 



No certain evidence that the a particles carried a charge 

 could be obtained. It was suggested that the failure to. 

 detect this charge might be due to the presence of a number 

 of slow moving electrons, liberated from the plates by the* 

 action of the rays. 



The following explanation (Joe. cit.) was given : — 



" The apparent absence of charge on the a particles would 

 be explained if an equal number of negatively charged 

 particles, or electrons, were expelled at the same time with a 

 slow velocity. If the electrons had about the same penetrating- 

 power as the u particles, it would be difficult to detect, their 



