of light could be obtained from highly electrified bodies at the moment when they were discharged. Such sparks are usually accompanied by a snapping sound, suggesting on a small scale the thunder accompanying the lightning spark, as was remarked by Newton and other early observers. Pale flashes of light are also produced by the discharge of electricity through tubes partially exhausted of air by the air-pump. Other effects will be noticed in due course. 12. Other Sources of Electrification. The student must be reminded that friction is by no means the only source of electrification. The other sources, percussion, compression, heat, chemical action, physiological action, contact of metals, etc., will be treated of in Lesson VII. We will simply remark here that friction between two different substances always produces electrical separation, no matter what the substances may be. Symmer observed the production of electrification when a silk stocking was drawn over a woollen one, though woollen rubbed upon woollen, or silk rubbed upon silk, produces no electrical effect. If, however, a piece of rough glass be rubbed on a piece of smooth glass, electrification is observed; and indeed the conditions of the surface play a very important part in the production of electrification by friction. In general, of two bodies thus rubbed together, that one becomes negatively electrical whose particles are the more easily removed by friction. Differences of temperature also affect the electrical conditions of bodies, a warm body being usually negative when rubbed on a cold piece of the same substance. The quantity of electrification produced is, however, not proportional to the amount of the actual mechanical friction; hence it appears doubtful whether friction is truly the cause of the electrification. Something certainly happens when the surfaces of two different substances are brought into intimate contact, which has the result that when they are drawn apart they are found (provided at least one of them is a non-conductor) to have acquired opposite charges of electrification; one surface having apparently taken some electricity from the other. But these opposite charges attract one another and cannot be drawn apart without there being mechanical work done upon the system. The work thus spent is stored up in the act of separating the charged surfaces; and as long as the charges remain separated they constitute a store of potential energy. The so-called frictional electric machines are therefore machines for bringing dissimilar substances into intimate contact, and then drawing apart the particles that have touched one another and become electrical. If the two bodies that are rubbed together are both good conductors, they will not become strongly electrified, even if held on insulating handles. It is quite likely, however, that the heat produced by friction, as in the bearings of machinery, is due to electric currents generated where the surfaces meet and slip. 13. Electric Field. . Whenever two oppositely charged surfaces are placed near one another they tend to move together, and the space between them is found to be thrown into a peculiar state of stress, as though the medium in between had been stretched. To explore the space between two bodies one of which has been positively and the other negaFig. 9. tively electrified, we may use a light pointer (Fig. 9) made of a small piece of very thin paper pierced with a hole through which passes a long thread of glass. It will be found that this pointer tends to point across from the positively electrified surface to the negatively electrified surface, along invisible lines of electric force. The space so filled with electric lines of force is called an electric field. In Fig. 8 A and B represent two bodies the surfaces of which have been electrified, the one positively, the other negatively. In the field between them the electric lines pass across almost straight, except near the edges, where they are curved. Electric lines of force start from a positively charged surface at one end, and end on a negatively charged surface at the other end. They never meet or cross one another. Their direction indicates that of the resultant electric force at every point through which they pass. The stress in the medium thus mapped out by the lines of force acts as a tension along them, as though they tended to shorten themselves. In fact in Fig. 8 the tension in the medium draws the two surfaces together. There is also a pressure in the medium at right angles to the lines, tending to widen the distance between them, Fig. 10 represents a ball which has been positively electrified, and placed at a distance from other objects; the lines in the field being simply radial. LESSON II. Electroscopes 1 ig. 10. 14. Simple Electroscopes. An instrument for detecting whether a body is electrified or not, and whether the electrification is positive or negative, is termed an Electroscope. The feather which was attracted or repelled, and the two pith-balls which flew apart, as we found in Lesson I., are in reality simple electroscopes. There are, however, a number of pieces of apparatus better adapted for this particular purpose, some of which we will describe. 15. Needle Electroscope. The earliest electroscope was that devised by Dr. Gilbert, and shown in Fig. 11, which consists of a stiff strip balanced lightly upon a sharp point. A thin strip of brass or wood, a straw, or even a goose quill, balanced upon a sewing needle, will serve equally well. When an electrified body is held near तु Fig. 11. the electroscope it is attracted and turned round, and will thus indicate the presence of electric charges far too feeble to attract bits of paper from a table. 16. Gold-Leaf Electroscope. - A still more sensi tive instrument is the Gold-Leaf Electroscope, invented by Bennet, and shown in Fig. 12. We have seen how two pith-balls when similarly electrified repel one another and stand apart, gravity being partly overcome by the force of the electric repulsion. A couple of narrow strips of the thinnest tissue paper, hung upon a support, will behave similarly when electrified. But the best results are obtained with two strips of gold leaf, which, being excessively thin, is much lighter than the thinnest paper. The Gold-Leaf Electroscope is conveniently made by suspending the two leaves within a wide-mouthed glass jar, which both serves to protect them from draughts of air and to support them from contact with the ground. The mouth of the jar should be closed by a plug of paraffin wax, through which is pushed a bit of varnished glass tube. Through this passes a stiff brass wire, the lower end of which is bent at a right angle to receive the two strips of gold leaf, while the upper supports a flat plate of metal, or may be furnished with a brass knob. When kept dry and free from dust it will indicate excessively small quantities of electrification. A rubbed glass rod, even while two or three feet from the instrument, will cause the leaves to repel one another. The chips produced by sharpening a pencil, falling on the electroscope top, are seen to be electrified. If the knob be even brushed with a small camel's hair brush, the slight friction produces a perceptible effect. With this instrument all kinds of friction can be shown to produce electrification. Let a person, standing upon an insulating support, - such as a stool with glass legs, or a board supported on four glass tumblers, be briskly struck with a silk handkerchief, or with a fox's tail, or even brushed with a clothes brush, he will be electrified, as will be indicated by the electroscope if he place one hand on the knob at the top of it. The Gold-Leaf Electroscope can further be used to indicate the kind of electrification on an excited body. Thus, suppose we rubbed a piece of brown paper with a piece of indiarubber and desired to find out whether the electrification excited on the paper was + or we should |