Page:Popular Science Monthly Volume 87.djvu/120

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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 difficulty, Rutherford and Geiger employed a method of magnifying automatically several thousand times the electric effect due to an alpha

PSM V87 D120 Apparatus for counting alpha particles.png

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 electrode 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 illustrates the results obtained:

Number of Throws Magnitude of Successive
Throws, Scale Divisions
1st minute 4 11, 12, 10, 11
2d minute 3 10, 11, 8
3d minute 5 10, 9, 13, 8, 12
4th minute 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.