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Fig. 2.
Fig. 3.
Fig. 2 shows that the intensity of the current becomes constant for high potential differences between the plates. Fig. 3 represents the same curves on another scale, and comprehends only relative results for small differences of potential. At the origin, the curve is rectilinear; the ratio of the intensity of the current to the difference of potential is constant for weak forces, and represents the initial conduction between the plates. Two important characteristic constants of the observed phenomenon are therefore to be recognised:—(1) The initial conduction for small differences of potential; (2) the limiting current for great potential differences. The limiting current has been adopted as the measure of the radio-activity.
Besides the difference of potential established between the two plates, there exists between them an electromotive force of contact, and these two sources of current combine their effects; for this reason, the absolute value of the intensity of the current changes with the sign of the external difference of potential. In every case, for considerable potential differences, the effect of the electromotive force of contact is negligible, and the intensity of the current is therefore the same whatever be the direction of the field between the plates.
The investigation of the conductivity of air and other gases subjected to the action of Becquerel rays has been undertaken by several physicists. A very complete research upon the subject has been published by Mr. Rutherford.
The laws of the conductivity produced in gases by the Becquerel rays are the same as those found for the Röntgen rays. The mechanics of the phenomenon appear to be the same in both cases. The theory of ionisation of the gases by the action of the Röntgen or Becquerel rays agrees well with the observed facts. This theory will not be put forward here. I will merely record the results to which they point:—
Firstly, the number of ions produced per second in the gas is considered proportional to the energy of radiation absorbed by the gas.
Secondly, in order to obtain the limiting current relatively to a given radiation, it is necessary, on the one hand, to cause complete absorption of this radiation by the gas by employing a sufficient mass of it; on the other hand, it is necessary for the production of the current to use all the ions generated by establishing an electric field of such strength that the number of the ions which recombine may be a negligible fraction of the total number of ions produced in the same time, most of which are carried by the current to the electrodes. The strength of the electric field necessary to give this result is proportional to the amount of ionisation.
According to the recent researches of Mr. Townsend, the phenomenon is more complex when the pressure of the gas is low. At first the current appears to approach to a constant limiting value with increasing difference of potential; but after a certain point has been reached, the current begins again to increase with the field, and with very great rapidity. Mr. Townsend ascribes this increase to a new ionisation produced by the ions themselves when, under the action of the electric field, they acquire a velocity such that a molecule of gas encountering one of them becomes broken down into its constituent ions. A strong electric field and a low pressure are favourable to the production of this ionisation by ions already present, and, as soon as the action is set up, the intensity of the current increases uniformly with the field between the plates. The limiting current could, therefore, only be obtained under conditions of ionisation of which the intensity does not exceed a certain value, and in such a manner that saturation corresponds to fields in which, from multiplicity of ions, ionisation can no longer take place. This condition has occurred in my experiments.
The order of magnitude of the saturation currents obtained with uranium compounds is 10–11 ampères for a condenser in which the plates have a diameter of 8 c.m., and are at a distance of 3 c.m. Thorium compounds give rise to currents of the same order of magnitude, and the activity of the oxides of uranium and thorium is very similar.
Radio-activity of the Compounds of Uranium and Thorium.
The following are the figures I obtained with different uranium compounds. I have represented the intensity of the current in ampères by the letter i:—
| i × 1011. | |
|---|---|
| Metallic uranium (containing a little carbon) | 2·3 |
| Black oxide of uranium, U2O5 | 2·6 |
| Green oxide of uranium, U3O4 | 1·8 |
| Hydrated uranic acid | 0·6 |
| Uranate of sodium | 1·2 |
| Uranate of potassium | 1·2 |
| Uranate of ammonium | 1·3 |
| Uranium sulphate | 0·7 |
| Sulphate of uranium and potassium | 0·7 |
| Nitrate of uranium | 0·7 |
| Phosphate of copper and uranium | 0·9 |
| Oxysulphide of uranium | 1·2 |
The thickness of the layer of the uranium compound used has little effect, provided that the layer is uniform. The following illustrate this point:—
| Thickness of layer. M.m. | i × 1011. | |
|---|---|---|
| Uranium oxide | 0·5 | 2·7 |
| Uranium oxide | 3·0 | 3·0 |
| Ammonium uranate | 0·5 | 1·3 |
| Ammonium uranate | 3·0 | 1·4 |
It may be concluded from this that the absorption of uranium rays by the substance which generates them is very great, since the rays proceeding from deep layers produce no significant effect.
The figures I obtained with thorium compounds enable me to state:—
Firstly, that the thickness of the layer used has considerable effect, especially in the case of the oxide.
Secondly, that the action is only regular if a sufficiently thin layer is used (e.g., 0·25 m.m.). On the contrary, when a thick layer of the substance is used (6 m.m.), the figures obtained vary between two extreme limits, especially in the case of the oxide:—
