In consequence of the importance of this question, a brief account will be given of the methods of measurement adopted and the special experimental difficulties which have arisen.
In the first place, it must be remembered that only a small fraction of the α rays, emitted from a layer of powdered radium bromide, escape into the surrounding gas. On account of the ease with which the α rays are stopped in their passage through matter, only those escape which are expelled from a superficial layer, and the rest are absorbed by the radium itself. On the other hand, a much larger proportion of the β rays escape, on account of their greater power of penetration. In the second place, the α particle is a far more efficient ionizer of the gas than the β particle, and, in consequence, if the charge carried by the α rays is to be determined by methods similar to those employed for the β rays (see section 80), the pressure of the gas surrounding the conductor to be charged must be very small in order to eliminate, as far as possible, the loss of charge resulting from the ionization of the residual gas by the α rays[[148]].
The experimental arrangement used by the writer is shown in [Fig. 33].
A thin film of radium was obtained on a plate A by evaporation of a radium solution containing a known weight of radium bromide. Some hours after evaporation, the activity of the radium, measured by the α rays, is about 25 per cent. of its maximum value, and the β rays are almost completely absent. The activity measured by the α and β rays is then slowly regained, and recovers its original value after about a month’s interval (see [chapter XI.]). The experiments were made on the active plate when its activity was a minimum, in order to avoid complications due to the presence of β rays. The film of radium was so thin that only a very small fraction of the α rays was absorbed.
Fig. 33.
The active plate A was insulated in a metal vessel D, and was connected to one pole of the battery, the other pole being earthed. The upper electrode, which was insulated and connected with a Dolezalek electrometer, consisted of a rectangular copper vessel BC, the lower part of which was covered with a thin sheet of aluminium foil. The α rays passed through the foil, but were stopped by the copper sides of the vessel. This arrangement was found to reduce the secondary ionization produced at the surface of the upper plate. The outside vessel D could be connected with either A or B or with earth. By means of a mercury pump, the vessel was exhausted to a very low pressure. If the rays carry a positive charge, the current between the two plates measured by the electrometer should be greater when A is charged positively. No certain difference, however, between the currents in the two directions was observed, even when a very good vacuum was obtained. In some arrangements, it was found that the current was even greater when the lower plate was negative than when it was positive. An unexpected experimental result was also noticed. The current between the parallel plates at first diminished with the pressure, but soon reached a limiting value which was not altered however good a vacuum was produced. For example, in one experiment, the current between the two parallel plates, placed about 3 mms. apart, was initially 6·5 × 10-9 amperes and fell off directly as the pressure. The current reached a limiting value of about 6 × 10-12 amperes, or about ¹⁄₁₀₀₀ of the value at atmospheric pressure. The magnitude of this limiting current was not much altered if the air was replaced by hydrogen.
Experiments of a similar character have been made by Strutt[[149]] and J. J. Thomson[[150]]; using an active bismuth plate coated with radio-tellurium (polonium) after Marckwald’s method. This substance emits only α rays, and is thus especially suitable for experiments of this kind. Strutt employed the method used by him to show the charge carried by the β rays ([Fig. 27]). He found, however, that, even in the lowest possible vacuum, the electroscope rapidly lost its charge and at the same rate whether it was charged positively or negatively. This is in agreement with the results found by the writer with radium.
In the experiments of J. J. Thomson, the electroscope was attached to a metal disc placed 3 cms. from the plate of radio-tellurium. A very low vacuum was produced by Dewar’s method by absorbing the residual gas in cocoanut charcoal immersed in liquid air. When the electroscope was charged negatively, an extremely slow rate of leak was observed, but when charged positively the leak was about 100 times greater. This showed that the polonium gave out large quantities of negative electricity, but not enough positive to be detected. By placing the apparatus in a strong magnetic field, the negative particles were prevented from reaching the electroscope and the positive leak was stopped.
These results indicate that these negative particles are not projected with sufficient velocity to move against the repulsion exerted by the electrified body, and are bent by a magnetic field. There thus seems little doubt that a stream of negative particles (electrons) is projected from the active surface at a very slow speed. Such low velocity electrons are also projected from uranium and radium. It is probable that these electrons are a type of secondary radiation, set up at the surfaces on which the α rays fall. The particles would be extremely readily absorbed in the gas, and their presence would be difficult to detect except in low vacua. J. J. Thomson at first obtained no evidence that the α particles of polonium were charged; but in later experiments, where the plates were closer together, the electroscope indicated that the α rays did carry a positive charge.