45. Conductivity of different gases when acted on by the rays. For a given intensity of radiation, the rate of production of ions in a gas varies for different gases and increases with the density of the gas. Strutt[[76]] has made a very complete examination of the relative conductivity of gases exposed to the different types of rays emitted by active substances. To avoid correction for any difference of absorption of the radiation in the various gases, the pressure of the gas was always reduced until the ionization was directly proportional to the pressure, when, as we have seen above, the ionization must everywhere be uniform throughout the gas. For each type of rays, the ionization of air is taken as unity. The currents through the gases were determined at different pressures, and were reduced to a common pressure by assuming that the ionization was proportional to the pressure.
With unscreened active material, the ionization is almost entirely due to α rays. When the active substance is covered with a layer of aluminium ·01 cm. in thickness, the ionization is mainly due to the β or cathodic rays, and when covered with 1 cm. of lead, the ionization is solely due to the γ or very penetrating rays. Experiments on the γ rays of radium were made by observing the rate of discharge of a special gold-leaf electroscope filled with the gas under examination and exposed to the action of the rays. The following table gives the relative conductivities of gases exposed to various kinds of ionizing radiations.
| Gas | Relative Density | α rays | β rays | γ rays | Röntgen rays |
|---|---|---|---|---|---|
| Hydrogen | 0·0693 | 0·226 | 0·157 | 0·169 | 0·114 |
| Air | 1·00 | 1·00 | 1·00 | 1·00 | 1·00 |
| Oxygen | 1·11 | 1·16 | 1·21 | 1·17 | 1·39 |
| Carbon dioxide | 1·53 | 1·54 | 1·57 | 1·53 | 1·60 |
| Cyanogen | 1·86 | 1·94 | 1·86 | 1·71 | 1·05 |
| Sulphur dioxide | 2·19 | 2·04 | 2·31 | 2·13 | 7·97 |
| Chloroform | 4·32 | 4·44 | 4·89 | 4·88 | 31·9 |
| Methyl iodide | 5·05 | 3·51 | 5·18 | 4·80 | 72·0 |
| Carbon tetrachloride | 5·31 | 5·34 | 5·83 | 5·67 | 45·3 |
With the exception of hydrogen, it will be seen that the ionization of gases is approximately proportional to their density for the α, β, γ rays of radium. The results obtained by Strutt for Röntgen rays are quite different; for example, the relative conductivity produced by them in methyl iodide was more than 14 times as great as that due to the rays of radium. The relative conductivities of gases exposed to X rays has been recently re-examined by McClung[[77]] and Eve[[78]], who have found that the conductivity depends upon the penetrating power of the X rays employed. The results obtained by them will be discussed later ([section 107]).
This difference of conductivity in gases is due to unequal absorptions of the radiations. The writer has shown[[79]] that the total number of ions produced by the α rays for uranium, when completely absorbed by different gases, is not very different. The following results were obtained:
| Gas | Total Ionization |
| Air | 100 |
| Hydrogen | 95 |
| Oxygen | 106 |
| Carbonic acid | 96 |
| Hydrochloric acid gas | 102 |
| Ammonia | 101 |
The numbers, though only approximate in character, seem to show that the energy required to produce an ion is probably not very different for the various gases. Assuming that the energy required to produce an ion in different gases is about the same, it follows that the relative conductivities are proportional to the relative absorption of the radiations.
A similar result has been found by McLennan for cathode rays. He proved that the ionization was directly proportional to the absorption of the rays in the gas, thus showing that the same energy is required to produce an ion in all the gases examined.
46. Potential Gradient. The normal potential gradient between two charged electrodes is always disturbed when the gas is ionized in the space between them. If the gas is uniformly ionized between two parallel plates, Child and Zeleny have shown that there is a sudden drop of potential near the surface of both plates, and that the electric field is sensibly uniform for the intermediate space between them. The disturbance of the potential gradient depends upon the difference of potential applied, and is different at the surface of the two plates.
In most measurements of radio-activity the material is spread over one plate only. In such a case the ionization is to a large extent confined to the volume of the air close to the active plate. The potential gradient in such a case is shown in [Fig. 9]. The dotted line shows the variation of potential at any point between the plates when no ionization is produced between the plates; curve A for weak ionization, such as is produced by uranium, curve B for the intense ionization produced by a very active substance. In both cases the potential gradient is least near the active plate, and greatest near the opposite plate. For very intense ionization it is very small near the active surface. The potential gradient varies slightly according as the active plate is charged positively or negatively.