The rate of decay has been more accurately determined by Rossignol and Gimingham[[233]] who found that the activity fell to half value in about 51 seconds. Bronson[[234]], using the steady deflection method described in section 69, found the corresponding time 54 seconds.
The decrease of the current with the time is an actual measure of the decrease of the activity of the emanation, and is not in any way influenced by the time that the ions produced take to reach the electrodes. If the ions had been produced from a uranium compound the duration of the conductivity for a saturation voltage would only have been a fraction of a second.
The rate of decay of the activity of the emanation is independent of the electromotive force acting on the gas. This shows that the radio-active particles are not destroyed by the electric field. The current through the gas at any particular instant, after stoppage of the flow of air, was found to be the same whether the electromotive force had been acting the whole time or had been just applied for the time of the test.
The emanation itself is unaffected by a strong electric field and so cannot be charged. By testing its activity after passing it through long concentric cylinders, charged to a high potential, it was found that the emanation certainly did not move with a velocity greater than ·00001 cm. per second, for a gradient of 1 volt per cm., and there was no evidence to show that it moved at all. This conclusion has been confirmed by the experiments of McClelland[[235]].
The rate at which the emanation is produced is independent of the gas surrounding the active matter. If in the apparatus of [Fig. 51] air is replaced by hydrogen, oxygen, or carbonic acid, similar results are obtained, though the current observed in the testing vessel varies for the different gases on account of the unequal absorption by them of the radiation from the emanation.
If a thorium compound, enclosed in paper to absorb the α radiation, is placed in a closed vessel, the saturation current due to the emanation is found to vary directly as the pressure. Since the rate of ionization is proportional to the pressure for a constant source of radiation, this experiment shows that the rate of emission of the emanation is independent of the pressure of the gas. The effect of pressure on the rate of production of the emanation is discussed in more detail later in [section 157].
142. Effect of thickness of layer. The amount of emanation emitted by a given area of thorium compound depends on the thickness of the layer. With a very thin layer, the current between two parallel plates, placed in a closed vessel as in [Fig. 17], is due very largely to the α rays. Since the α radiation is very readily absorbed, the current due to it practically reaches a maximum when the surface of the plate is completely covered by a thin layer of the active material. On the other hand the current produced by the emanation increases until the layer is several millimetres in thickness, and then is not much altered by adding fresh active matter. This falling off of the current after a certain thickness has been reached is to be expected, since the emanation, which takes several minutes to diffuse through the layer above it, has already lost a large proportion of its activity.
With a thick layer of thorium oxide in a closed vessel, the current between the plates is largely due to the radiation from the emanation lying between the plates. The following tables illustrate the way in which the current varies with the thickness of paper for both a thin and a thick layer.
Table I. Thin Layer.
Thickness of sheets of paper ·0027.