Physical actions.
119. Some electric effects. Radium rays have the same effect as ultra-violet light and Röntgen rays in increasing the facility with which a spark passes between electrodes. Elster and Geitel[[198]] showed that if two electrodes were separated by a distance such that the spark just refused to pass, on bringing near a specimen of radium the spark at once passes. This effect is best shown with short sparks from a small induction coil. The Curies have observed that radium completely enveloped by a lead screen 1 cm. thick produces a similar action. The effect in that case is due to the γ rays alone. This action of the rays can be very simply illustrated by connecting two spark-gaps with the induction coil in parallel. The spark-gap of one circuit is adjusted so that the discharge just refuses to pass across it, but passes by the other. When some radium is brought near the silent spark-gap, the spark at once passes and ceases in the other[[199]].
Hemptinne[[200]] found that the electrodeless discharge in a vacuum tube began at a higher pressure when a strong preparation of radium was brought near the tube. In one experiment the discharge without the rays began at 51 mms. but with the radium rays at 68 mms. The colour of the discharge was also altered.
Himstedt[[201]] found that the resistance of selenium was diminished by the action of radium rays in the same way as by ordinary light.
F. Henning[[202]] examined the electrical resistance of a barium chloride solution containing radium of activity 1000, but could observe no appreciable difference between it and a similar pure solution of barium chloride. This experiment shows that the action of the rays from the radium does not produce any appreciable change in the conductivity of the barium solution.
Kohlrausch and Henning[[203]] have recently made a detailed examination of the conductivity of pure radium bromide solutions, and have obtained results very similar to those for the corresponding barium solutions. Kohlrausch[[204]] found that the conductivity of water exposed to the radiations from radium increased more rapidly than water which had not been exposed. This increase of conductivity may have been due to an increase of the conductivity of the water itself, or to an increased rate of solution of the glass of the containing vessel.
Specimens of strongly active material have been employed to obtain the potential at any point of the atmosphere. The ionization due to the active substance is so intense that the body to which it is attached rapidly takes up the potential of the air surrounding the active substance. In this respect it is more convenient and rapid in its action than the ordinary taper or water dropper, but on account of the disturbance of the electric field by the strong ionization produced, it is probably not so accurate a method as that of the water dropper.
120. Effect on liquid and solid dielectrics. P. Curie[[205]] made the very important observation that liquid dielectrics became partial conductors under the influence of radium rays. In these experiments the radium, contained in a glass tube, was placed in an inner thin cylinder of copper. This was surrounded by a concentric copper cylinder, and the liquid to be examined filled the space between. A strong electric field was applied, and the current through the liquid measured by means of an electrometer.
The following numbers illustrate the results obtained:
| Substance | Conductivity in megohms per 1 cm.3 |
|---|---|
| Carbon bisulphide | 20 × 10-14 |
| Petroleum ether | 15 „ |
| Amyline | 14 „ |
| Carbon chloride | 8 „ |
| Benzene | 4 „ |
| Liquid air | 1·3 „ |
| Vaseline oil | 1·6 „ |
Liquid air, vaseline oil, petroleum ether, amyline, are normally nearly perfect insulators. The conductivity of amyline and petroleum ether due to the rays at -17° C. was only ⅒ of its value at 0° C. There is thus a marked action of temperature on the conductivity. For very active material the current was proportional to the voltage. With material of only ¹⁄₅₀₀ of the activity, it was found that Ohm’s law was not obeyed.
The following numbers were obtained:
| Volts | Current |
|---|---|
| 50 | 109 |
| 100 | 185 |
| 200 | 255 |
| 400 | 335 |
For an increase of voltage of 8 times, the current only increases about 3 times. The current in the liquid thus tends to become “saturated” as does the ordinary ionization current through a gas. These results have an important bearing on the ionization theory, and show that the radiation probably produces ions in the liquid as well as in the gas. It was also found that X rays increased the conductivity to about the same extent as the radium rays.
Becquerel[[206]] has recently shown that solid paraffin exposed to the β and γ rays of radium acquires the property of conducting electricity to a slight extent. After removal of the radium the conductivity diminishes with time according to the same law as for an ionized gas. These results show that a solid as well as a liquid and gaseous dielectric is ionized under the influence of radium rays.
121. Effect of temperature on the radiations. Becquerel[[207]], by the electric method, determined the activity of uranium at the temperature of liquid air, and found that it did not differ more than 1 per cent. from the activity at ordinary temperatures. In his experiments, the α rays from the uranium were absorbed before reaching the testing vessel, and the electric current measured was due to the β rays alone. P. Curie[[208]] found that the luminosity of radium and its power of exciting fluorescence in bodies were retained at the temperature of liquid air. Observations by the electric method showed that the activity of radium was unaltered at the temperature of liquid air. If a radium compound is heated in an open vessel, it is found that the activity, measured by the α rays, falls to about 25 per cent. of its original value. This is however not due to a change in the radio-activity, but to the release of the radio-active emanation, which is stored in the radium. No alteration is observed if the radium is heated in a closed vessel from which none of the radio-active products are able to escape.
122. Motion of radium in an electric field. Joly[[209]] found that a disc, one side of which is coated with a few milligrams of radium bromide, exhibits, when an electrified body is brought near it, motions very different to those observed in the case of an inactive substance. The electrified body, whether positive or negative, repels the suspended body if brought up to it on the side coated with radium, but attracts it if presented to the naked side.
This effect is very simply shown by constructing a small apparatus like a radiometer. Two covered glasses are attached to the end of a glass fibre about 6 cms. long, the surfaces lying in the same plane. The apparatus is free to rotate on a pivot. The two vanes are coated on alternate faces with radium bromide, and the whole apparatus contained within a glass receiver. If an electrified rod of ebonite or sealing wax is brought up close to the receiver, a rotation is communicated to the vane which increases as the pressure of the air is lowered to 5 or 6 cms. of mercury. By placing the apparatus between parallel plates connected with the terminals of a Wimshurst machine, a steady rotation is communicated to the vanes. The rotation is always in such a direction that the radium coated surface is repelled from the electrified body.
This action was examined still further by attaching the vanes to the glass beam of a Coulomb’s balance. A metal sphere, which could be charged from without, was fixed facing the side coated with radium. A repulsion was always observed except when the charge was very strong and the vane near the sphere. If, however, the two vanes were connected by a light wire and a similar sphere placed exactly opposite the other, an attraction was observed if one sphere was charged, but a repulsion if both were charged. These effects were observed whether the vanes were of aluminium or glass.
Joly found that the effect could not be explained by any direct action due to the movement of the ions in an electric field. The recoil, due to the expulsion of α particles from one side of the vane, is far too small to account for the movement observed.
This effect can, I think, be simply accounted for by taking into consideration the difference in conductivity of the gas on the two sides of the radium coated vane. If a small vane, coated uniformly with radium on both sides, and mounted on an insulating support, be brought near a charged body kept at a constant potential, it acts like a water dropper and rapidly acquires very nearly the average potential which existed at that point before the vane was brought up. The mechanical force acting on the vane will, in consequence, be small. If, however, the vane is only coated with radium on the side near the charged body, the ionization and consequently the conductivity of the gas is much greater between the vane and the charged body than on the opposite side. Suppose, for simplicity, the body is charged to a positive potential. On account of the greater conductivity of the gas on the side facing the charged body, it will rapidly acquire a positive charge, and the potential of the vane will reach a higher value than existed at that place before the vane was introduced. This will result in a repulsion of the vane. This also accounts for the attraction observed in the experiment with the Coulomb’s balance already referred to. Suppose that one sphere is positively charged and the other earthed, and the two vanes metallically connected together. The vane next to the charged body will become charged positively, but this charge will be dissipated rapidly on account of the ionization of the gas close to the opposite vane, and, in most conditions, this loss of charge will be so rapid that the potential of the vane is unable to reach the value which would exist at that place in the field, if the vane were removed. There will, in consequence, be an attracting force acting on the vane towards the sphere.
The repulsion observed by Joly is thus only an indirect result of the ionization in the gas produced by the radium, and should be shown under conditions where similar unequal distribution of ionization is produced by any other sources.
Since radium gives out heat at a fairly rapid rate, a radiometer in which the vanes were coated on one side with radium instead of lampblack, should rotate at low pressure of the gas, even if no source of light is brought near it. This should evidently be the case, since the face coated with radium should reach a slightly higher temperature than the other. This experiment has been tried, but the effect seems too small to produce rotation of the vanes.