[m'lennan] on electric CHARGES 83 



TJiis then is one type of charge which is nearly always acquired by 

 such an insulated system as that described. It has, however, certain de- 

 finite characteristics which make it readily detectable. For example, the 

 rapidity with which this charge is acquired depends largely on the degree 

 of conductivit}' possessed by the intervening gas, and as this conductivity 

 can be increased at will by bringing more or less near to the vessel a 

 small quantity of radium or other radioactive body, it is possible to make 

 the insulated system practically take up at once a charge which will 

 suffice to annul the volta effect. 



If, further, now the inner vessel or body in the arrangement de- 

 scribed possesses in addition a radiating surface which emits alpha or 

 beta rays, the charging action of these rays can generally be brought into 

 evidence by a reduction of the pressure of the gas between the two bodies. 

 This reduction of the pressure will produce a diminution in the conduc- 

 tivity of the gas, but it will not affect the rate of emission of charged, 

 particles from the radiating surface. Consequently as the pressure of 

 the gas is reduced the insulated system should, under the action of its 

 charged radiation, acquire a charge, and so set up a potential difference 

 between the inner and outer vessels. 



The actual current through a gas, however, at any particular pres- 

 sure depends, so long as the saturation current has not been attained, 

 upon the potential difference producing the current, and so it happens 

 that at each pressure a state of equilibrium is brought about by virtue of 

 which the insulated system is maintained at such a potential as will pro- 

 duce a current through the gas of such magnitude as to exactly counter- 

 balance the gain of charge through the emission of the radiation. 



But as the pressure of the gas is lowered the equilibrium potential 

 of the insulated system becomes gradually greater and greater, and the 

 sign of the charge on the free system corresponding to tliis gradually in- 

 creasing potential difference will be the opposite of that of the emitted 

 radiation which is in excess. 



Further, the extent of the equilibrium potential corresponding to 

 any particular pressure will give a measure of the magnitude of this 

 excess radiation. 



Wlien applying the method to the investigation of any particular 

 radiation, the earth connection to the insulated system should first be 

 broken and time allowed for the free system to come into equilibrium 

 under the action of the conduction current arising from the volta effect. 

 When this equilibrium has been reached the scale reading corresponding 

 to the position then assumed by the movable system may therefore be 

 taken as the initial reading in considering the charging action due to 

 the radiation itself. Two lines of procedure are then open. If the ra- 



