Gold-Leaf Electroscope is one of the instruments used for testing positive and negative charges. It consisted of a metal rod, A to which gold leaves, L were attached (See image below).
The rod was fitted with a circular disc or cap B; and was translated with a plug P from a metal case C which screened L from outside influences other than those bought near to B.
When B is touched by an ebonite rob rubbed with fur, some of the negative charge on the rod passed to the cap and L; and since like charges repel, the leaves diverge (Fig below(i)). If an unknown charge X is now brought near to B, an increased divergence implies that X is negative (Fig. below (ii)). A positive charge is tested in a similar way; the gold-leaf electroscope is first given a positive charge and an increased divergence indicates a positive charge.
We shall now show that it is possible to obtain charges, called induced charges, without any contact with another charge. An experiment on electrostatic induction, as the phenomenon is called, is shown in Fig. below(i). Two insulated metal spheres A, B are arranged so that they touch one another, and negatively charged ebonite rod C is brought near to A. The spheres are now separated, and then the rod is taken away. Tests with a charged pith-ball now show that A has a positive charge and B a negative charge (Fig. below(ii)). If the spheres are placed together so that they touch, it is found that they now have no effect on a pith-ball held near. Their charges must therefore have neutralized each other completely, thus showing that the induced positive and negative charges are equal. This is explained by the movement of electrons from A to B when the rod is brought near. B has then a negative charge and A an equal positive charge.
Charging by Induction
The Fig below shows how a conductor can be given a permanent charge by induction, without dividing it on two. We first bring a charged ebonite rod, say, near to the conductor, (i); next we connect the conductor to earth by touching it momentarily (ii); finally we remove the ebonite. We then find that the conductor is left with a positive charge (iii). If we use a charged glass rod, we find that the conductor is left with a negative charge; the charge left, called the induced charge, has always the opposite sign to the inducing charge.
This phenomenon of induction can again be explained by the movement of electrons. If the inducing charge is negative, then when we touch the conductor, electrons are repelled from it to earth, as shown inducing in Fig. below(ii), and a positive charge is left on the conductor. If the inducing charge is positive, then the electrons are attracted up from the earth to the conductor, which then becomes negatively charged.
Induction and the Gold-Leaf Electroscope
It is always observed that the leaves of an electroscope diverge when a charged body is brought near its cap, without touching it. This we can now easily understand; if, for example, we bring a negatively charged rod near the cap, it induces a positive charge on the cap, and a negative one on the leaves: the leaves repel each other. Further, the negative charge on the leaves induces a positive one on the inside of the case, the corresponding negative charge running to the earth, on which the case rests. The positive charge on the case attracts the negative charge on the leaves, and makes them diverge further.
We can use induction to give a permanent charge to the capo and leaves of a gold-leaf electroscope, by momentarily earthing the cap while holding an inducing charge near it.
The electrophorus is a device which provides an almost unlimited supply of charge, by induction, was invented by Volta about 1800; it is called an electrophorus. It consists of an ebonite or Perspex base, E in ( Fig. below), and a metal disc D on an insulating handle. The ebonite is charged negatively by rubbing it – or, much better, beating it – with fur. The disc is then laid upon it, and acquires induced charges, positive underneath and negative on top, (i). Very little negative charge escapes from the ebonite to the disc, because the natural unevenness of their surfaces prevents them touching at more than a few points; charge escapes from these points only, because the ebonite is a non-conductor. After it has been placed on the ebonite, the disc is earthed with the finger, and the negative charge on its upper surface flows away, (ii). The disc can then be removed and carries with it the positive charge which was on its underside, (iii).
An electrophorus produces sufficient charge to give an audible – sometimes a visible – spark. The disc can be discharged and charged again repeatedly, until the charge on the ebonite has disappeared by leakage. Apparently, therefore, it is in principle an inexhaustible source of energy. However, work is done in raising the disc from the ebonite, against the attraction of their opposite charges, and this work must be done each time disc is charged; the electrophorus is therefore not a source of energy, but a device for converting it from a mechanical into an electrical form.
The action of the electrophorus illustrates the advantages of charging by induction. First, the supply of charge is almost inexhaustible, because the original charge is not carried away. Second, a great charge – nearly equal to the charge on the whole ebonite – can be concentrated on to the conducting disc. As we have seen, only a very small charge could be transferred by contact, because the ebonite is not a conductor.