Page:Radio-activity.djvu/231

 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 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:

++-+ ++-+ ++-+
 * | Conductivity in    |
 * Substance      | 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 1/10 of its value at 0° C. There is thus a marked action of temperature on the conductivity. For very active material the current was