I give below the results of three other experiments with different combinations:—
| Original E.M.F. |
New value after the action of radiation |
Percentage reduction. | |
| I……… | 1.26 V. | 1.15 V. | 9 |
| II……… | 0.39 V. | 0.312 V. | 20 |
| III……… | 0.065 V. | 0.039 V. | 40 |
I was now desirous of obtaining a continuous record of electromotive changes produced by the continued action of electric radiation. For this purpose I used a galvanometer.
A cell was made, in the way previously described, with magnesium powder. Owing to some differences in the two portions of the powder there was an initial P.D. of 0·042 V. between the two electrodes. I now had the cell balanced by the potentiometer method, a sensitive galvanometer (with an interposed high resistance) being used as the detector of electric variation. Fig. 8 (a) shows the deviation from the balanced position by radiation which nearly reduced the potential difference to half its original value. By tapping, the original P.D. was restored, and a second experiment (see (b) ) gave almost identical results.
It is thus seen that the curve of electromotive variation due to radiation is similar to that obtained by the conductivity variation method.
It has already been shown that when the range of electric elasticity of the substance is not narrow, or when the strain is not too great, there is a recovery. That on subjecting the substance to the continued action of radiation there is a limiting effect; that too long continued action tends to produce an electric reversal; that too feeble an intensity may also produce a reversed effect. We shall now study whether visible radiation produces similar effects.
