2 Rutherford, a Particles from Radio-active Matter. 



with an ordinary electrometer. This is done by making 

 use of the property discovered by Tovvnsend, that an ion 

 moving in a strong electric field in a gas at low pressure, 

 produces a number of fresh ions by collision with the gas 

 molecules. If the electric field is adjusted nearly to the 

 value required for the passage of the spark, a single ion 

 generated in the gas by external agencies, produces in 

 this way several thousand fresh ions by collision. In the 

 experimental arrangement, the testing vessel consists of 

 a brass tube 60 cms. in length, along the axis of which 

 passes a thin insulated wire attached to the electrometer. 

 With a gas pressure of about 2 cms, a potential difference 

 of about 1,000 volts between the brass tube and the wire 

 is required. The a particles are fired down the tube 

 through a small hole at the end of the tube about 2 mms. 

 in diameter covered with a thin plate of mica. In order 

 to use a narrow pencil of a rays, the active matter in the 

 form of a thin film on a surface about one square cm; in 

 area is placed in an exhausted tube which is a prolongation 

 of the testing vessel. The distance of the active matter 

 from the hole is usually between 50 and 75 cms. and the 

 amount of active matter adjusted so that from six to ten 

 a particles are fired through the hole per minute. The 

 effect of the a particle entering the testing vessel is 

 shown by a sudden throw of the electrometer needle. 

 Under good conditions, this throw is about 50 divisions 

 using an electrometer which has a sensibility of 300 

 divisions per volt. By observing the number of throws 

 of the electrometer needle, we can count the average 

 number of a particles shot through the opening per 

 minute. The total number fired out by the active 

 matter can be calculated from the known area of the 

 opening and the distance of the latter from the active 

 matter. Preliminary observations show that the number 



