Page:Radio-activity.djvu/344

 *nation takes place. This must obviously be the case, since the positive carriers travel further in a half alternation towards A than they return towards B during the next half alternation. The carriers thus move backwards and forwards in the changing electric field, but on the whole move towards the plate A. The total number of positive carriers produced between the plates during a complete alternation is 2dqT. The ratio ρ of the number which reach B to the total number produced is thus given by ρ = ((1/2)x_{1}qT + (1/2)x_{1}(x_{1}/x_{2})qT)/(2dqT) = (1/4)(x_{1}/d)(x_{1} + x_{2})/x_{2}. Substituting the values of x_{1} and x_{2}, we find that K = (2(E_{0} + E_{1}))/(E_{0}(E_{0} - E_{1}))(d^2/T)ρ. In the experiments, the values of E_{0}, E_{1}, d, and T were varied, and the results obtained were in general agreement with the above equation. The following were the results for thorium:  ''Plates 1·30 cms. apart.''

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 * E_{0} + E_{1}|E_{0} - E_{1}|Alternations|ρ|K | |                |                 | per second |     |    | +-+-++-++ |       152       |       101       |     57     | ·27 |1·25| |       225       |       150       |     57     | ·38 |1·17| |       300       |       200       |     57     | ·44 |1·24| +-+-++-++   Plates 2 cms. apart.

+-+-++-++ +-+-++-++ +-+-++-++
 * E_{0} + E_{1}|E_{0} - E_{1}|Alternations|ρ|K |
 * |                | per second |     |    |
 * 273      |       207       |     44     | ·37 |1·47|
 * 300      |       200       |     53     | ·286|1·45|

The average mobility K deduced from a large number of experiments was 1·3 cms. per sec. per volt per cm. for atmospheric