486 MAGNETISM AND ELECTRICITY an excess of protons. It is a fundamental concept of our modern electrical theory of matter that opposite charges attract each other, whereas like charges repel each other. It is easy to explain by the modern electron theory the phenomena just observed. According to this theory, glass, when rubbed by silk, loses electrons and therefore becomes positively charged. On the other hand, hard rubber, when rubbed by wool, gains electrons from the wool and therefore becomes negatively charged. The pith ball, when touched by the glass rod, is repelled because it gives up some of its electrons to the glass rod; and, thus becoming positively charged, the ball is repelled by the glass rod, which still possesses an excess of protons. The concept that electrostatic fields surround all electric charges is very useful in explaining Fig. 229. The lines of , ^, • • ,. rr r 1 . c J many phenomena. 1 he imagmary Imes oi force • I force in an electric held ,. . . ,, ,• • • r between two opposite radiating in all directions from a single electric charges. charge, either positive or negative, terminate on other opposite charges. Figure 229 shows the field between two opposite charges. The conception of tension along these lines and of a compression at right angles to them, explains electric attraction and repulsion, just as it served to explain magnetic attraction and repulsion using magnetic lines of force. A Static Charge May Be Induced by Bringing a Charged Body Near an Uncharged Body. Perhaps you have already raised the question : why was the pith ball originally attracted to the charged rod? This is explained by assuming that the charges on the pith ball corresponding to the charge on the rod were driven to that surface of the pith ball farthest away from the rod, thus leaving an excess of opposite charges on the nearest side. This idea can well be illustrated by the electroscope. The electroscope is an instrument used to identify and indicate the charge on a body. One form of the electroscope consists of a pair of gold leaves suspended in a metal box to shut out air drafts, and provided with windows to look through. If a charged rod is brought near the electroscope, the leaves will repel each other because they acquire a like charge by induction. This charge is not permanent, however, for the electrons have only shifted their position in the leaves and have not been transferred to another object. When the charged rod is removed, the leaves fall back next to each other.
STATIC ELECTRICITY 487 If the outside knob of the electroscope connected directly with the leaves is touched by the charged rod and the rod is again removed, the leaves diverge and do not fall back again. If one places his finger on the knob and then removes it while the charged rod is still in contact with the knob, the leaves will fall; but when the rod is removed the leaves will diverge and remain apart for some time. In this case, the charge induced on the leaves has been transferred to the body, with the result that the leaves have an excess or deficiency of electrons, as the case may be. An electroscope can thus be charged with a known charge and be used to determine the charge on another body, because a like charge brought near the electroscope will produce greater deflection, whereas an unlike charge will decrease the deflection. Only Insulators or Insulated Bodies Can Be Charged. Fig. 230. A gold leaf electroscope. If one tries to charge metals by rubbing them, he will find upon testing them with the electroscope that they cannot be charged as long as they are held in the hand but that they can be charged when held by glass or hard-rubber handles because metals conduct electricity, while glass and hard rubber do not. Substances which conduct electricity are called conductors, while those which do not conduct electricity are called nonconductors, or insulators. The electrons in the atoms of nonconductors are thought to be held too tightly within the atoms to move freely, while the electrons in conductors are relatively free to move. All metals are good conductors, although some are better conductors than others. Silver, copper, gold, and aluminum, in the order given, are the best conductors. Sulfur, a nonmetal, is a very poor conductor. Charges Distribute Themselves on the Surfaces of Objects. Charges are found to be distributed on the surfaces of objects. The electrical force of repulsion between like charges causes them to get as far away from each other as possible. The force between these charges varies inversely as the square of the distance between them. If a charged rod is brought near two conductors mounted on insulating bases and in contact with each other, electrons will be driven to one