There are several ways in which the air may become ionized. The different rays given off by radioactive substances (Alpha, Beta, and Gamma rays) all have the power of driving off electrons from the molecules of gases; i. e., ionizing them. Air is undoubtedly ionized by radioactive matters in the soil (radium and thorium) and especially by the gaseous “emanations” of these substances in the atmosphere, which are also radioactive. It has, however, been a problem to account for ionization over the ocean; because the amount of radioactive matter in sea water is immeasurably small, while the amount of radioactive emanation in sea air is, according to the latest observations, only about 2.5 per cent of that occurring over the land.
The clue to this mystery seems to be found in a special kind of Gamma rays coming from some region far above the surface of the earth. These rays are called the “penetrating radiation,” because they not only are able, like the Gamma rays due to radioactive substances on earth, to pass through the walls of hermetically sealed metal vessels and ionize the air inside, but they also have the power of passing through a great extent of atmosphere without being absorbed. They are estimated to be about ten times as “penetrating” as the Gamma rays coming from known terrestrial substances. The best proof that a radiation of this sort comes from above is that when closed metal vessels are carried up in balloons, there is, above an altitude of about half a mile, a rapid increase in the rate at which ions are produced within them. As to the source of this radiation, one suggestion has been that it comes from strongly radioactive cosmic dust in the upper atmosphere. A hypothesis that seems more plausible at present attributes it to the bombardment of the atmosphere by electrons shot off from the sun.
The knowledge of ions in the atmosphere is one of the recent acquisitions of science. On the other hand, it has been known for some generations that the earth has normally a negative charge as compared with the air, or the air a positive charge as compared with the earth. Thus between the earth and any point in the air (except, as we now know, at great altitudes) there is a difference of what is called “potential,” of such a character that negative electricity will follow any conductor provided for it away from the earth. Variations of potential with altitude have long been measured by means of instruments called “collectors,” which gather, so to speak, a sample of the electrical charge of the air at any point and enable it to be compared with that of the earth. The difference of potential is measured in volts per meter of vertical distance. Thus we get the “potential gradient,” which averages about 150 volts per meter in the lowest part of the atmosphere. It is subject to great irregular variations—especially during thunderstorms—and also to somewhat regular rises and falls during each day, and to an annual fluctuation, being much greater in winter than in summer.
It has also been known for a good many years that the air is a conductor of electricity—though a poor one—and, therefore, does not insulate the earth. Dr. W. F. G. Swann has expressed the extremely small conductive capacity of the air for electricity in the statement that a column of it one inch long offers as much resistance to the passage of an electrical current as a copper cable, of the same cross section, thirty thousand million million miles long!
A fact more recently learned, from observations in balloons, is that the potential gradient falls off rapidly at high levels, and becomes practically zero at an altitude of about six miles. From this fact it is concluded that the lower six miles of the atmosphere contains a charge of positive electricity just equal to the negative charge at the earth’s surface. In other words, the lower atmosphere is not only positive with respect to the earth, but in an absolute sense it contains an excess of positive electricity.
Thus we have a negatively charged earth surrounded by a layer of positively charged air. Since air is a conductor, it is not easy to see why the opposite charges of the earth and the atmosphere do not combine and neutralize each other. An interchange is, in fact, always going on between them; negative ions flow upward from the earth and positive ions flow in the opposite direction. This “earth-air current” is, however, exceedingly small. Moreover, the opposite charges of the earth and air remain from year to year in spite of it.
How does the earth retain its negative charge and the air its positive charge? No other question relating to atmospheric electricity has, in recent years, been so much debated as this. Discussion centers, as a rule, upon the negative charge of the earth; for there are certain reasons for assuming that, when once this is explained, the positive charge of the atmosphere will require no special explanation.
One hypothesis is that the earth is bombarded by positive and negative corpuscles from the sun, and that the negative corpuscles have such penetrating power that they are able to reach the earth, while the positive corpuscles are caught by the atmosphere. Another hypothesis (Swann’s) is that the same “penetrating radiation” that, as we have seen, helps to ionize the lower atmosphere has the effect of driving downward a stream of electrons detached from the air molecules, thus maintaining a constant supply of negative electricity to the earth. The question is not yet settled.
It is now time to turn from these somewhat abstruse matters to a subject of universal interest; viz., lightning. As recently as a few decades ago, though there was already a copious literature on the subject of lightning, very little was really known about it. Even its superficial features were strikingly misunderstood until the advent of photographic methods of investigation. Thus until the middle of the nineteenth century sharply zigzag lightning flashes were represented in scientific books as they still are in conventional art. That so-called zigzag lightning is really sinuous was first asserted by James Nasmyth, in 1856, and his contention was soon afterward confirmed by photography. The camera has revealed a large number of other interesting things about lightning.
Everybody has noticed an appearance of flickering in lightning flashes that are of sensible duration. Several early investigators, such as Arago, Dove, and O. N. Rood, had reached the conclusion that such flashes are multiple, consisting of several successive discharges along an identical path. Rough measurements of the intervals of time between these discharges were made with various forms of rotating disk. Far more accurate information is now obtained on this subject by the use of a camera mounted on a vertical axis and swung in a wide arc, at a fixed rate, by clockwork. The perfection of this device is due, in part, to A. Larsen, in America, but especially to Dr. B. Walter, of Hamburg, whose achievements in the photography of lightning far surpass those of any other investigator.