CHAPTER VII.

ELECTRICAL SEPARATION.

LESSON XXXVI.-DEVELOPMENT OF ELECTRICITY.

335. IT was known as early as six centuries B. C. that when amber is rubbed with silk it attracts light bodies, and Dr. Gilbert, in the sixteenth century, showed that many other substances, such as sulphur, sealing-wax, and glass, possess similar properties.

From this very small beginning our knowledge of these phenomena has of late years vastly increased, and we know that this attractive power manifested by rubbed bodies is the result of the development of an agent which we term electricity (from the Greek word "λekтpov, amber).

336. Conductors and Insulators.-Suppose we have a metal rod with a glass stem, and rub the glass with a piece of flannel, the glass will in consequence have the power of attracting light bodies, but only at that place where it has been rubbed. Thus the property which the glass has acquired has not the power of spreading itself over its surface. Now we may by various means communicate the same properties to the metal rod to which the glass is attached, and we shall find that the influence has spread over the whole surface of the metal, and is not localized as in the former case. This fact we express by saying that glass is an insulater, or non-conductor of electricity, and that a metal is a conductor of it.

Accordingly bodies have been divided into two classes,

as far as electricity is concerned, and the following table exhibits the place in which they stand :

The transition from the first to the second class of bodies is not altogether abrupt, but the worst kinds of conductors are to some extent insulators, and even the very best conductor presents some resistance to the passage of electricity.

On the other hand, various circumstances may render a body a conductor; thus glass heated to a red heat is a conductor, although when cold, glass does not conduct.

It is likewise of great importance to make all our experiments on electricity in a dry atmosphere; because, if the air be charged with vapour, particles of water will be deposited on the surface of the apparatus employed, tending to conduct away the electricity.

Indeed the necessity to us of insulators in the study of electricity will readily appear, for if all bodies were conductors, it is evident that we could not confine electricity so as to perform any experiments upon it.

337. Electricity is of two kinds.-Let us suspend a small pith ball by means of a silk thread to a glass support, as in Fig. 109, forming what is called an electrical pendulum. Let us now, in the first place, rub a glass rod with silk, and then make the glass rod so rubbed touch the pith ball. After contact has taken place, the pith ball will be repelled by the glass rod. If now we rub a stick of sealing-wax with a piece of flannel or cloth, and bring it near the pith ball, the latter will be attracted to the excited sealing-wax.

Thus a pith ball touched with excited glass will be afterwards repelled by excited glass, but attracted by excited sealing-wax.

In like manner, if we had touched the pith ball with excited sealing-wax instead of excited glass, it would thereafter have been repelled by excited sealing-wax, but attracted by excited glass.

FIG. 109.

This experiment shows us that there are two kinds of electricity, namely, that which we obtain from excited glass, and which we may call vitreous, and that from excited sealing-wax, which we may call resinous electricity. We see also that when the pith ball has been touched by excited glass, and thus receives part of its electricity, it is thereafter repelled by the glass, and in like manner if it be touched by excited sealing-wax it is thereafter repelled by the sealingwax, and hence we conclude that bodies charged with similar electricities repel one another.

On the other hand, the pith ball, if charged with excited glass, will be attracted to excited sealing-wax, and if charged with excited sealing-wax it will be attracted to excited glass, and hence we conclude that bodies charged with opposite electricities attract one another.

338. The Hypothesis of two Fluids. It is not meant here to speculate upon the nature of electricity, but for convenience' sake we may regard it as a fluid of which there are two opposite kinds, vitreous and resinous, or, as they are more frequently called, positive and negative. According to this hypothesis every substance may be supposed to contain an indefinite quantity of these two electricities mixed together, and neutralizing one another. By various means the two fluids may be separated the one from the other, but whenever we have a certain amount of positive electricity, there must be somewhere else just as much negative electricity. Therefore in rubbing together two bodies such as sealing-wax and a piece of cloth we do not produce a certain quantity of negative electricity by itself, but the sealing-wax becomes negatively electrified, while the cloth on which it is rubbed becomes positively electrified.

In the following table each substance will be positively electrified if rubbed by any substance that follows it in the list, but negatively if rubbed by any substance that precedes it :

I. Cat's skin

2. Flannel

3. Glass

4. Cotton

5. Silk

6. Wood

7. Shellac

8. Resin 9. Metals

10. Sulphur

II. Caoutchouc

12. Gutta-percha

339. Other modes of developing Electrical Separation.— There are other methods of developing electricity besides that produced by rubbing two bodies together, for it has been noticed that when heterogeneous substances are pressed together, and then suddenly separated from each other, electrical excitement is frequently produced. It has also been noticed by Becquerel that cleavage frequently produces electrical separation, as for instance when two plates of mica are rapidly torn from each other.

Volta was the first to suppose that electrical separation is produced by the contact of heterogeneous metals, and the

truth of this has lately been demonstrated by Sir W. Thomson. We shall return to this when we come to treat of the electric current. In all these cases there is a heterogeneity or difference between the two substances or portions of the same substance between which electricity is produced, and it is believed that if two absolutely similar substances were brought together, and then separated or rubbed against each other, we should not be able to obtain any electrical separation.

We must also bear in mind that electrical separation requires energy, so that when an electrical machine is in good action, part of the work spent in turning it is converted into heat, and part into electrical separation.

In fact, in some way or another we must always have spent energy before we can produce electrical separation.

Besides the strictly mechanical sources of electricity there are certain minerals which, when heated, exhibit electrical properties, in which case they are said to be pyroelectric.

Tourmaline is a mineral of this kind. It it not the absolute temperature, but only the change of temperature that renders tourmaline electric. Thus if a tourmaline be in a room of any temperature, and if it is completely in temperature equilibrium with this room, it will not be electric. But if taken into a colder medium, it acquires two contrary electric poles, which however vanish when the tourmaline has acquired the new temperature. If it now be brought into the original hotter medium, the tourmaline again acquires electrical poles, but in the opposite direction. Suppose we call these two poles A and B. When taken from the hotter medium into the colder, let us suppose that the pole A is positive and B negative, then when taken from the colder medium and transferred once more to the hotter medium, A will become negative and B positive. It may be asked, what species of energy is spent in this case to produce the electrical separation? to which the reply is, that heat is spent; a small portion of the heat has in fact vanished in producing this separation, and will again appear in the shape of heat when we have recombined the two electricities.