212 Prof. Rutherford on Dependence of the Current through 



]ike radium, where the ionizing-power due to the radiation is 

 mainly confined to within a lew centimetres of the active 

 surface. Some experiments with Rontgen-rays will be given 

 later, but, on account of the inconstancy of a Rontgen-tube, 

 it was found more convenient and accurate to experiment 

 with radio-active substances. 



Fig. 1 shows the general arrangement of the experiment 

 with radio-active substances. 



Two parallel circular insulated plates of lead A and B, 



Fisr. 1. 



Earth 



Sarth 



19 centims. in diameter, were fixed horizontally inside a tin 

 vessel C. The lower plate was covered with a layer of 

 tinfoil on which a thin layer of radium was lightly sprinkled. 

 The upper plate was also covered with tinfoil and rigidly 

 attached to the lid of the vessel. The central portion D of 

 the upper plate was separated from the outer by a narrow 

 air-gap and insulated from it, so that the outer served as a 

 guard-ring. The electrometer was connected to the central 

 plate in the usual manner ; the guard-ring and vessel were 

 connected to earth ; the lower plate to one terminal of a 

 large battery of small accumulators, the other terminal of 

 which was to earth. The crossed lines in the figure represent 

 insulators. The current between the plates was measured 

 by noting the deflexion of the electrometer-needle in a given 

 time. 



The quadrants were separated, and the connexion with the 



