for the pointed wire connected with the inner surface gives such great facilities for the escape of the electric fluid, that a jar cannot

LECTRICITY, &c.

ELEMENTARY LECTURES ON ELECT

LECTURE XII.

There is such a variety of experiments which might be brought forward by the employment of the Leyden jar, most of which are highly favourable to the Franklinean theory, that there is not so much difficulty in finding illustrations, as in knowing where to discontinue them in proper time; but as we are yet within bounds, I will offer a few more to your notice.

I will uncover the ball of the Leyden jar, and expose the pointed extremity of the wire which is connected with the metallic lining to the prime conductor, by bringing them into contact with one another. Now, as the machine is kept in action, the jar will charge in the usual way; and now that it is charged, I place it on a sheet of tin foil on the table. I now touch the tin foil with one hand, and with the back part of the other hand I approach the point of the projecting wire; and I experience an agreeable aura, or electric breeze, gently brushing against my hand. It now slackens its intensity, which, indeed, seems to diminish very rapidly. It has now ceased altogether, so that I may venture to touch the wire with my hand without danger of receiving a shock. Now in this experiment the jar has been discharged silently, and without any appearance of light, which were consequences of the discharge being accomplished gradually, through the medium of the points of the wire, and keeping the hand at a suitable distance: for had I brought my hand close to the wire suddenly, whilst the jar was fully charged, I should have received a violent blow in my arms and shoulders, as decidedly as if the wire had been surmounted with a ball.

This is a very instructive experiment, and may be varied in several different ways. If, for instance, when the jar is charged, I employ the discharging rod, instead of my hand, towards the point of the vertical wire, the discharge will be accomplished silently and without light, as decidedly as by the application of the hand; but in both cases, when either the hand or the ball of the discharging rod is brought sufficiently near to the point of the jar's wire, a singular crackling noise will be produced, and if the room be darkened, a thin crooked stream of exceedingly minute sparks will be observed. If the hand be applied for this purpose, a singular disagreeable prickling sensation is experienced on the skin; but the crackling noise is best produced by the ball of the discharging rod.

Another mode of varying the experiment is, that of first charging the jar, and then placing it on the table without any further manipulation. In the course of a few minutes afterwards the jar will have disposed of the whole of the charge into the surrounding air: for the pointed wire connected with the inner surface gives such great facilities for the escape of the electric fluid, that a jar cannot

possibly retain its charge many minutes under such circumstances. If the jar, when charged, be placed in a darkened room, you may have ocular evidence of the discharging process, by observing a beautiful brush of attenuated purple electric light issuing from the point of the wire into the atmosphere: and not only that fact will be observed, but another, of still greater interest. The electric brush, which first appears, whilst the jar is highly charged, is much larger, and by far more elegant, than during any other period of the exhibition; the reason of which is, that as the discharging process itself is occasioned by the repulsive forces amongst the congregated particles of the fluid on the inner surface of the jar, and as those particles are more numerous, and, consequently, their combined forces more powerful, at first, than at any other time, they then spring with the greatest degree of energy through the point into the air; and in proportion as the discharge goes on, the electric forces within the jar abate, and eventually become too feeble to drive off any more of the electric matter.

This property of metallic points of throwing the electric fluid into the surrounding medium, is sometimes taken advantage of in the charging process; and is, indeed, another mode of illustrating this property, as well as being also a peculiar mode of illustrating a certain part of the theory of the jar. Let us hang the jar on the wire of the prime conductor of the machine, as in fig. 1. plate vi: but instead of the second jar applied to the coating of the first, we will furnish that coating with a point. If now the room be darkened, and the machine be put to work, you will see an electric pencil of light going from the pointed wire attached to the coating of the jar. Now as this is the case, and as a jar charges when the fluid naturally belonging to its outer surface can make its escape, it is plain that the jar ought now to be charged; so that by applying the discharging rod in the usual way, we shall find that such is the case; for a bright flash of light and a smart report is immediately produced.

As it is not my intention, in these elementary lectures, to advance facts in direct opposition to the statements of other writers on this subject, for the mere purpose of refutation, I shall pass such statements unnoticed, and proceed as though they had never been made.

I will first shew you an experiment with the jar placed at some distance from the prime conductor, directly below it, and with the pointed wire directed upwards. When the machine is put into motion, the jar begins to receive a charge as decidedly as if the point had been placed in close contact with the prime conductor; and when the machine has been in action for a few seconds, we shall find the charge almost as high, if not quite so, as under any other circumstances. An application of the discharging rod immediately indicates this fact. Again, we will now place the jar on a support, so that the point shall be on a level with the axis of the primel conductor, and at the distance of two feet from it: the charging process will

go on as in the former instance, but not quite so rapidly; yet the charge thus attained will be tolerably high.

The question now is, did the jar in both these cases, or in either of them, receive the charge from the prime conductor? Let us try to decide this point by another experiment. For this purpose

we will employ another neat little apparatus, represented by fig. 5, plate vi., and called an electrometer. It was invented by Mr. Henly, about the year 1770, and as the original description of it is given at the end of this lecture, I shall not say anything more about it in this place, further than that it is applied to the prime conductor by being stuck in a hole made for its reception on the upper side. The pith ball, b, terminating the index, is repelled from the main stem as the charge of a jar advances, and will sometimes attain a deflection of more than 90°, or, in other words, the ball b will rise to above the height of the hinge, or centre of motion. And, on the other hand, should the prime conductor, to which the instrument is attached, suffer any loss of fluid whilst charged, the decline of the index deflection immediately shows it.

Having now made you acquainted with the use of the electrometer, we will proceed to experiment, by first fixing the instrument in the prime conductor, and placing the jar at a distance beneath it; but we had better satisfy ourselves, in the first place, that the machine is in good action, and that the index of the electrometer stands high when the jar is not present. Having now found the machine in good action, by the index of the electrometer forming an angle with the lower part of the stem, of more than 90°, I will place the jar beneath the conductor, with its pointed wire at a distance of more than twelve inches, still keeping the machine in motion. Now let me solicit your attention to the electrometer, whose index has fallen full 50°, indicating a decline in the electric intensity of the primal conductor. I will now remove the jar for a moment, and you see that the index of the electrometer mounts up again as high as at first, which is a sufficient evidence to shew that no decline of energy in the machine has taken place; and therefore we may rest satisfied that, whilst the jar was present, the decline of the index of the electrometer did not arise from any corresponding decline in the action of the machine; and, therefore, the loss of intensity in the prime conductor was due to some other cause. Now we have already seen, by the previous experiment, that the jar can be charged by being placed in the same situation as in this; and as the only difference in the method of performing these experiments consists in the additional electrometer in the present case, we have only to try if the jar can be charged whilst that instrument is attached to the prime conductor. A few turns of the machine, and a proper application of the discharging rod, satisfy us that the jar can be charged as decidedly by the presence of the electrometer as without it. Now, from all these circumstances we have no other legitimate inference than that the jar received its