Page:Elementary Text-book of Physics (Anthony, 1897).djvu/403

§ 330] Faraday showed that many of the phenomena of the discharge depend to some extent upon the medium in which it occurs. The differences in color and in the facility with which various forms of the discharge were set up in the gases upon which he experimented were especially noticeable.

It was proved by Franklin that the lightning flash is an electrical discharge between a cloud and the earth or another cloud at a different electrical potential. The differences of potential to which such discharges are due must be enormous, and the heat developed by the discharge shows that the quantity of electricity which passes in it is considerable.

Slowly moving fire-balls are sometimes seen, which last for a considerable time and disappear with a loud report and with all the attendant phenomena of a lightning discharge. It is probable that they are glow discharges which appear just before the difference of potential between the cloud and the earth becomes sufficiently great to give rise to a lightning flash.

320. The Electrical Discharge in Rarefied Gases.—If the air between the electrodes of an electrical machine be heated, it is found that the discharge takes place with greater facility and that the spark which can be obtained is longer than befuie. Similar phenomena appear if the air about the electrodes be rarefied by means of an air-pump. After the rarefaction has reached a certain point the discharge ceases to pass as a spark, and becomes apparently continuous. The arrangement in which this discharge is studied consists of a glass tube into which are sealed two platinum or, preferably, aluminium wires to serve as electrodes, and from which the air is removed to any required degree of exhaustion by an air-pump. Such an arrangement is usually called a vacuum-tube.

As the exhaustion proceeds there appears about the negative electrode in the tube a bright glow, separated from the electrode by a small non-luminous region. The body of the tube is filled with a faint rosy light, which in many cases breaks up into a succession of bright and dark layers transverse to the direction of the discharge. The discharge in this case is called the stratified discharge. A