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With the modern machines large sparks are produced by merely turning a crank, enough electricity being produced to imitate a small thunderstorm. The sparks of home-made lightning will jump several inches.

Do not think that electricity is generated in a commercial way by static electric machines. The practical uses of static electricity are very few when compared with those of current electricity from batteries and dynamos.

15. Condensation of Static Electricity. By means of apparatus called condensers, a terrific charge of static electricity may be stored. Fig. 11 shows the most common form of condenser, known as the Leyden jar. It consists of a glass jar with an inside and outside coating of tin-foil.

Fig. 11.

Fig. 12.

To charge the jar it is held in the hand so that the outside coating shall be connected with the earth, the sparks from an electric machine being passed to the knob at the top, which is connected by a chain to the inside coating.

To discharge the jar, Fig. 12, a conductor with an insulating handle is placed against the outside coat; when the other end of the conductor is swung over towards the knob, a bright spark passes between them. This device is called a discharger. Fig. 13 shows a discharge through ether which the spark ignites.

Fig. 13.

16. The Leyden Battery, Fig. 14, consists of several jars connected in such a way that the area of the inner and outer coatings is greatly increased. The battery has a larger capacity than one of its jars. (For Experiments in Condensation, see "Study," Chapter X.)

Fig. 14.

Fig. 15.

17. Electromotive Force of Static Electricity. Although the sparks of static electricity are large, the quantity of electricity is very small. It would take thousands of galvanic cells to produce a spark an inch long. While the quantity of static electricity is small, its potential, or electromotive force (E. M. F.), is very high. We say that an ordinary gravity cell has an E. M. F. of a little over one volt. Five such cells joined in the proper way would have an E. M. F. of a little over five volts. You will understand, then, what is meant when we say that the E. M. F. of a lightning flash is millions of volts.

18. Atmospheric Electricity. The air is usually electrified, even in clear weather, although its cause is not thoroughly understood. In 1752 it was proved by Benjamin Franklin (Fig. 15), with his famous kite experiment, that atmospheric and frictional electricities are of the same nature. By means of a kite, the string being wet by the rain, he succeeded, during a thunderstorm, in drawing sparks, charging condensers, etc.

Fig. 16.

19. Lightning may be produced by the passage of electricity between clouds, or between a cloud and the earth (Fig. 16), which, with the intervening air, have the effect of a condenser. When the attraction between the two electrifications gets great enough, a spark passes. When the spark has a zigzag motion it is called chain lightning. In hot weather flashes are often seen which light whole clouds, no thunder being heard. This is called heat lightning, and is generally considered to be due to distant discharges, the light of which is reflected by the clouds. The lightning flash represents billions of volts.

Fig. 17.

20. Thunder is caused by the violent disturbances produced in the air by lightning. Clouds, hills, etc., produce echoes, which, with the original sound, make the rolling effect.

21. Lightning-Rods, when well constructed, often prevent violent discharges. Their pointed prongs at the top allow the negative electricity of the earth to pass quietly into the air to neutralize the positive in the cloud above. In case of a discharge, or stroke of lightning, the rods aid in conducting the electricity to the earth. The ends of the rods are placed deep in the earth, Fig. 17.

22. St. Elmo's Fire. Electrification from the earth is often drawn up from the earth through the masts of ships, Fig. 18, to neutralize that in the clouds, and, as it escapes from the points of the masts, light is produced.

Fig. 18.

23. Aurora Borealis, also called Northern Lights, are luminous effects, Fig. 19, often seen in the north. They often occur at the same time with magnetic storms, when telegraph and telephone work may be disturbed. The exact cause of this light is not known, but it is thought by many to be due to disturbances in the earth's magnetism caused by the action of the sun.

Fig. 19.

CHAPTER II.
ABOUT MAGNETS AND MAGNETISM.

Fig. 20

24. Natural Magnets. Hundreds of years ago it was discovered that a certain ore of iron, called lodestone, had the power of picking up small pieces of iron. It was used to indicate the north and south line, and it was discovered later that small pieces of steel could be permanently magnetized by rubbing them upon the lodestone.

25. Artificial Magnets. Pieces of steel, when magnetized, are called artificial magnets. They are made in many forms. The electromagnet is also an artificial magnet; this will be treated separately.

26. The Horseshoe