electricities are not separated from each other, but mixed together; however, when you push them up to A, the positive electricity of A attracts the negative electricity of B to its side, and repels the positive away to the extreme right of c, as you see in the figure.

If we now pull c away from B, and finally pull B away from A, we shall thus have got a quantity of negative electricity in B, and a quantity of positive in c, both separate from each other, while the electricity in a will be the same as before.

We have, in fact, made use of the electricity in a to separate part of the two electricities of B and C from each other, and A is still as ready as ever to help us again. Now this distant action or help, rendered by the electricity of A in separating that of в and c, is called electric induction.

Α

80. The electric spark.-We may, however, perform our experiment in a somewhat different manner. Let us now bring B and C slowly towards A, and continue to do so. When and B are very near together, we shall have the positive electricity of A and the negative electricity, which has been induced to appear on B, separated from each other by only a small thickness of air until at last they will be so strong and the film of air so thin, that they will rush together and unite in the form of a spark. The consequence will be that A will have lost a portion of its positive electricity, and B will have lost all its negative. If we now pull в and c away there will still be the positive charge at c, which has not gone away; in fact, while a has lost part of its positive electricity, c will have gained just as much, so that the result is virtually the same as if part of the electricity of a had gone over to C

A

A

81. Sundry experiments.-What we have now aid about electric induction may be easily illustrated by a few simple and striking experiments; but it must be remembered that in all these experiments the glass of the apparatus must be quite dry and warm.

Fig. 41.

EXPERIMENT 55.-Here you see in the figure an instrument by which we can detect electricity, called the gold leaf electroscope. In order to show you its action, let me first of all communicate to the knob at the top (see Appendix) a slight charge of positive electricity. Now this charge runs to the gold leaves which are electrically connected with the knob, and then these leaves, being both charged with the same kind of electricity, begin to repel each other as you see in the figure. The electroscope is now in action.

EXPERIMENT 56.-Having thus charged the electroscope with positive electricity, let us bring near its knob an excited glass rod, when the gold leaves will diverge still more. The reason of this is that the positive electricity of the excited glass decomposes the neutral electricity of the knob attracting the negative to itself, and repelling the positive to the gold leaves. If, therefore, the leaves had been previously charged with positive electricity, they will now diverge more widely.

EXPERIMENT 57.—If we now bring near the knob of the electroscope, charged as before with positive electricity, a stick of excited sealing-wax, we shall first find that the gold leaves will collapse instead of diverging. The reason is that the negative electricity of the excited sealing-wax decomposes the neutral electricity of the knob attracting the positive to itself, and driving the negative to the gold leaves. But since the gold leaves were previously charged with positive electricity, part of this charge will be cancelled by the negative electricity driven towards them, and they will consequently collapse.

EXPERIMENT 58.-Here we have a hollow brass ball or conductor, supported on an insulating glass stand. Let us now bring this insulated conductor near the electric machine when in action, and we shall get a spark, but it will be very feeble. Let us now touch with our finger that part of the hollow ball which is farthest from the machine, and the spark given to the ball will now be much more intense.

This illustrates what we said in Art. 80 about the cause of the spark. In fact, the positive electricity of the machine pulls towards itself the negative electricity of the hollow ball, and drives away the positive as far as possible. If, however, this ball is insulated, the positive cannot be driven away sufficiently far, nor the two electricities be separated sufficiently well, and the consequence is you have but a feeble spark. But when you touch the hollow brass ball, the positive electricity of the ball is driven through your body to the earth, the electricities are thus well separated, and there is a good spark.

82. Action of points.-In the last experiment,

if you continue to touch the brass ball, and the electric inachine is worked at the same time, a succession of sparks will pass through your body to the earth, and these will cause you to feel a somewhat unpleasant sensation. The spark from the electric machine may in truth be compared to a flash of lightning-a flash of lightning being, in fact, a very long spark. Now, just as when a man is struck by lightning the electricity passes through his body to the earth, so when we grasp or touch the ball of the last experiment, the electricity goes through our body to the earth.

EXPERIMENT 59.—Now let us attach a point to the hollow ball, and place this point next the conductor of the machine, touching the ball as before with our finger. It will now be impossible to get a spark from the machine, but there will be instead a continuous rush of electricity. In fact, anything pointed carries off the electricity just as rapidly as it is produced, and does not give it time to gather so as to form a spark. We now see the use of the pointed metallic conductors that are placed above lofty buildings, to protect them from lightning strokes. These pointed metallic conductors, running down into the earth, carry off the electricity in a silent manner, just as the point did in Experiment 59; and just as the point protected my finger from a spark in the one case, so does the lightning conductor protect the building from a flash or stroke of lightning in the other.

Franklin, an American philosopher, was the first to find out that lightning and electricity are the same thing -the only difference being that a flash of lightning is often several miles in length, whereas an electric spark is only a few inches.

83. Electrical Machine.-You are now in a position to understand the construction of an electric machine. Such a machine is composed of two parts; we have first of all an arrangement for producing electricity, and we have next an arrangement for collecting it.

One of the best known machines is that in which the electricity is produced by a large plate of glass revolving, as in fig. 42. As the plate of glass

revolves, it is rubbed against by two sets of rubbers, one above and the other below. These rubbers are usually made of leather stuffed with horse-hair, so as to press rather tightly against the glass. They are coated with a soft metal, which is spread over the leather, and this metal is generally made of one part of zinc, one of tin, and two of mercury melted