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THE POPULAR SCIENCE MONTHLY 



particles called ions. The ionization due to the alpha particles can be 

 readily measured by electrical methods, and it can be shown that the 

 effect to be expected from a single alpha particle is much too small to 

 detect except by very refined methods. In order to overcome this diffi- 

 culty, Eutherford and Geiger employed a method of magnifying auto- 

 matically several thousand times the electric effect due to an alpha 



Fig. 3. Apparatus for Counting Alpha Particles. 



particle. The general arrangement of the original apparatus is seen 

 in Fig. 3. 



A few of the alpha rays from a radioactive substance passed along 

 an exhausted tube E through an opening D covered with thin mica into 

 the detecting tube AB. The latter contained a central insulated elec- 

 trode B connected with an electrometer, and the pressure of the gas 

 inside was adjusted to a few centimeters of mercury. The tube B was 

 connected with the negative pole of a battery of about 1,500 volts, the 

 other pole being earthen. The potential was adjusted so that a spark 

 was on the point of passing between A and B. Under such conditions, 

 the ionization due to an alpha particle passing along the detecting vessel 

 is magnified several thousand times by collision of the negative and 

 positive ions with the neutral molecules. 



The entrance of an alpha particle into the detecting vessel is then 

 signified by a sudden ballistic throw of the electrometer needle, and 

 the number of particles entering the vessel in a given time can be 

 counted by observing the throws. The amount of active matter and 

 its distance from the opening were adjusted so that three to five alpha 

 particles entered the opening per minute. The following table illus- 

 trates the results obtained: 



Magnitude of Successive 

 Number of Throws Throws, Scale Divisions 

 1st minute 4 11,12,10,11 



2d minute 3 10,11,8 



3d minute 5 10,9,13,8,12 



4thminute 4 18*, 8, 12 



5th minute 3 10, 6, 10 



6th minute 4 9,10,12,11 



7th minute 2 10,11 



8th minute 3 11,13,8 



9th minute 4 8,20 



10th minute 3 8,12,14,6 



Average per minute, 3.5. Average throw, 10 divisions. 



