ELECTRICITY, Atmospheric. Experi ments have shown that there is always free electricity in the atmosphere, which is some times negative and sometimes positive, but most generally positive, and the intensity of this free electricity is gteater in the middle of the day than at morning or night and is greater in win ter than in summer. In fine weather the poten tial increases with altitude at the rate, accord ing to some writers, of about 30 volts per foot. To detect the presence of free electricity in the air a pointed metal rod projecting into the air several feet and connected at its lower end to a gold leaf electroscope may be used. When this rod is projected into the air a few feet the leaves diverge. Kites and balloons have also been used to detect and, so to speak, draw down the free electricity of the air. The origin of atmospheric electricity is still unknown. Some physicists have ascribed it to the friction of the air upon the ground, others to the gradual oxidation of plant and animal life, others again to evaporation, to induction from the sun, and to differences of temperature. Most authori ties are agreed, however, that whatever may be the origin of free electricity in the atmos phere the electricity of enormous voltages that disrupts the air and produces the phenomena of lightning (q.v.) is due to the condensation
of the watery vapor forming the clouds; each minute vapor drop as it moves through the air collecting upon its surface a certain amount of free electricity. Then as these drops of vapor coalesce into larger drops with a corresponding decrease in the total surface exposed the elec tric potential rises until it overcomes the resist ing power of the air. This remark will be more clearly understood when it is considered that with a given charge of electricity its potential rises as the electrical capacity of the object holding the charge is decreased, which is the case when the minute vapor drops coalesce into larger drops.. The similarity of lightning, to the electricity developed by an electrical machine was demonstrated by Franklin in his memorable kite experiments.
Saint Elmo's fire (q.v.) is another phase of atmospheric electricity to be considered in this connection. It is otherwise known as the fire of Saint Elias, of Saint Clara, of Saint Nicho las and of Helena, as well as composite, corn posant or corposant (that is, corpus sancium). The phenomenon is observed usually during a thunder storm, at the tops of trees, spires, etc., or on the heads of animals, as a brush or star of light.