“Of the millions of tons of air which form the atmosphere nearly the whole is moving. The regions of calm at the surface at any one time, taken all together, do not form a large part of the earth’s surface, and above the surface calm regions are still rarer. Let us remember that the motion of the air is always ‘circulation’; air cannot move forward or backward or upward or downward without displacing other air in front of it and being replaced by other air behind it, though the circulation may be quite local and limited in extent, as is frequently the case when warm air rises or cold air sinks. In the course of investigations into the life history of surface air currents in the Meteorological Office we have traced air over long stretches of the surface of the Atlantic. We have found, on one occasion, the shores of Greenland to be fed with air that left the middle of the Atlantic four days previously, while in the course of six days air traveled from Spitsbergen to join the northeast trade wind off the west coast of Africa. On another occasion the air that formed the wind off the south of Ireland was traced back to the north of Africa, but that which blew at the opening of the Channel two days later came from Hudson Bay, via the Azores.”
Such are the ever-shifting currents of the ocean of air.
CHAPTER IX
ATMOSPHERIC ELECTRICITY
Every schoolboy has read how Benjamin Franklin, by means of his famous kite experiment, demonstrated the electrical nature of lightning, and how the same versatile genius invented the lightning rod. It is not proposed to repeat familiar history here. Neither shall we discuss the dubious statements frequently put forth that lightning rods were known before Franklin’s time, nor consider how much credit is due the many philosophers who, at earlier periods, suspected lightning to be a manifestation of electricity. The facts and ideas concerning atmospheric electricity that we have to present in this chapter were, for the most part, quite unknown to Franklin and to many generations of savants after him, and some of them are just now finding their way into the textbooks.
Science still recognizes the existence of two kinds of electricity—positive and negative—which, by combining, neutralize each other’s effects. According to current ideas, however, the more active agent in electrical phenomena is negative electricity, which is believed to consist of (or to provide electrical charges for) exceedingly minute particles called electrons.
Only a few years ago the smallest thing that science had to deal with was the atom, and the lightest of atoms is that of hydrogen. The discovery of electrons marks a new step toward the infinitely little. The mass of the electron—or, in more popular and less exact terms, its weight—is about 1/1800 that of a hydrogen atom. As to its size: Imagine a billiard ball magnified to the size of the earth. Its constituent atoms would be the actual size of billiard balls, but the electrons of which each atom is composed would still be too small to be seen with the naked eye. Now imagine each of these billiard-ball atoms further magnified to the size of a large church. The electrons would then be about as big as one of the periods on this page.
When we say that a body has an electrical charge we mean that it has an excess of positive or negative electricity. An ordinary molecule of an atmospheric gas contains (or perhaps actually consists of) equal amounts of the two kinds of electricity, and is therefore not charged. There are, however, various ways in which an electron may be detached from such a molecule, leaving it positively charged; and again it may receive an extra electron, and thus acquire a negative charge. Under the former circumstances it becomes a positive ion, and under the latter a negative ion. Ions play a very important role as carriers of electricity, because they are impelled to move toward oppositely charged bodies or particles and combine with them. A gas containing ions is said to be ionized; and it is the ionization of the atmosphere that makes it a conductor of electricity.
The number of ions in a given volume of air has been the subject of a great many measurements, both at observatories on land and in the course of scientific expeditions at sea. There are ingenious instruments called “ion counters,” in which air is drawn at a measured rate through the apparatus and its electrical effects are noted. The number of positive ions found in a cubic centimeter of the lower atmosphere varies from a few hundred to a thousand or more, while the number of negative ions in the same space is generally one or two hundred smaller. The ionization is about the same over the ocean as over the land.