Eve also observed that the amount of activity to be obtained per unit length of the wire in the zinc cylinder of about 70 cms. in diameter was about the same as for a wire ·5 mms. in diameter charged to 10,000 volts in the open air, supported 20 feet from the ground. This shows that such a potential does not draw in the carriers of excited activity which are more than half a metre away, and probably the range is even less.
It is of great importance to find how large a proportion of the number of ions produced in the atmosphere is due to the radio-active matter distributed throughout it. The results of Eve with the large iron tank, already referred to, indicate that a large proportion of the ionization in the tank was due to the radio-active matter contained in it, for the ratio of the excited activity on the central electrode to the total ionization current in the tank was about ⁷⁄₁₀ of the corresponding ratio for a smaller tank into which a supply of the radium emanation had been introduced.
This result requires confirmation by experiments at other parts of the earth, but the results point to the conclusion that a large part, if not all, of the ionization at the earth’s surface is due to radio-active matter distributed in the atmosphere. A constant rate of production of 30 ions per second per c.c. of air, which has been observed in the open air at the surface of the earth in various localities, would be produced by the presence in each c.c. of the air of the amount of emanation liberated from 2·4 × 10-15 grams of radium bromide in radio-active equilibrium. It is not likely, however, that the ionization of the upper part of the atmosphere is due to this cause alone. In order to explain the maintenance of the large positive charge, which generally exists in the upper atmosphere, there must be a strong ionization of the upper air, which may possibly be due to ionizing radiations emitted by the sun.
282. Ionization of atmospheric air. A large number of measurements have been made during the last few years to determine the relative amount of ionization in the atmosphere in different localities and at different altitudes. Measurements of this character were first undertaken by Elster and Geitel with a special type of electroscope. A charged body exposed to the air was attached to a portable electroscope, and the rate of loss of charge was observed by the movement of the gold or aluminium leaf. The rates of discharge of the electroscope for positive and negative electricity were generally different, the ratio depending on the locality and the altitude, and on the meteorological conditions. This apparatus is not suitable for quantitative measurements and the deductions to be drawn from the observations are of necessity somewhat indefinite.
Ebert[[430]] has designed a portable apparatus in which the number of ions per c.c. of the air can be determined easily. A constant current of air is drawn between two concentric cylinders by means of a fan actuated by a falling weight. The inner cylinder is insulated and connected with an electroscope. Knowing the capacity of the apparatus, and the velocity of the current of air, the rate of movement of the gold-leaf affords a measure of the number of ions present in unit volume of the air drawn between the cylinders.
In this way Ebert found that the number of ions in the air was somewhat variable, but on an average corresponded to about 2600 per c.c. in the particular locality where the measurements were made.
This is the equilibrium number of ions present per c.c. when the rate of production balances the rate of recombination. If q is the number of ions produced per second per unit volume of the air and n is the equilibrium number, then q = αn2 where α is the constant of recombination ([section 30]).
By a slight addition to the apparatus of Ebert, Schuster[[431]] has shown that the constant of recombination for the particular sample of air under investigation can be determined. The value so obtained for air in the neighbourhood of Manchester was variable, and two or three times as great as for dust-free air. The results of some preliminary measurements showed that the number of ions present per c.c. of the air in different localities varied from 2370 to 3660, while the value of q, the number of ions produced per c.c. per second, varied between 12 and 38·5.
Rutherford and Allan and Eberts showed that the ions in the air had about the same mobility as the ions produced in air by Röntgen rays and radio-active substances. In some recent determinations by Mache and Von Schweidler[[432]], the velocity of the positive ion was found to be about 1·02 cms. per second, and that of the negative 1·25 cms., for a potential gradient of one volt per cm.
Langevin[[433]] has recently shown that in addition to these swift moving ions, there are also present in the atmosphere some ions which travel extremely slowly in an electric field. The number of these slowly moving ions in the air in Paris is about 40 times as great as the number of the swifter ions. This result is of great importance, for in the apparatus of Ebert these ions escape detection, since the electric field is not strong enough to carry them to the electrodes during the time of their passage between the cylinders.