Thomson&rsquos Electrometer

Professor W. Thomson, of Glasgow, has devised an atmospheric electrometer, which is likely to become eminently successful, in the hands of skilful observers. It is mainly a torsion balance combined with a Leyden-jar. The index is an aluminium needle strung on a fine platinum wire, passing through its centre of gravity, and stretched firmly between two points. The needle and wire are carefully insulated from the greater part of the instrument, but are in metallic communication with two sma l plates fixed beside the two ends of the needle, and termed the repelling plates. A second pair of larger plates face the repelling plates, on the opposite side of the needle, but considerably farther from it. These plates are in connection with the inner coating of a Leyden-jar, and are termed the attracting plates. The whole instrument is enclosed in a metal cage, to protect the glass Leyden-jar and the delicate needle.

The Leyden-jar should be charged when the instrument is used. Its effect is two-fold: it increases greatly the sensibility of the instrument, and enables the observer to distinguish between positive and negative electrification.

The air inside the jar is kept dry by pumice-stone, slightly moistened with sulphuric acid; by which means very perfect insulation is maintained.

Electrodes, or terminals, are brought outside the instrument, by which the Leyden-jar can be charged, and the needle system connected with the body, the electric state of which is to be tested.

For the purpose of testing the electric state of the atmosphere, the instrument is provided with a conductor and support for a burning match, or, preferably, with an arrangement termed a water-dropping collector; by either of which means the electricity of the air is conveyed to the needle system.

The needle abuts upon the repelling plates when not influenced by electricity, in which position it is at zero. It can always be brought back to zero by a torsion-head, turning one end of the platinum wire, but insulated from it, and provided with a graduated circle, so that the magnitude of the arc, that the torsion-head is moved through to bring the needle to zero, measures the force tending to deflect it.

The action of the instrument is as follows:—The Leyden-jar is to be highly charged, say negatively; and the repelling plates are to be connected with the earth. The needle will then be deflected against a stop, under the combined influence of attraction from the Leyden-jar, or attracting plates, and repulsion from the repelling plates due to the positive charge induced on the needle and its plates by the Leyden-jar plates. The platinum wire must then be turned round by the torsion-head so as to bring back the needle to zero; and the number of degrees of torsion required will measure the force with which the needle is attracted. Next, let the needle plates be disconnected from the earth, and connected with the insulated body, the electric state of which is to be tested. In testing the atmosphere, the conductor and lighted match, or water-dropping apparatus, must be applied.

If the electricity of the body be positive, it will augment the positive charge in the needle plates, induced by the Leyden-jar plates; and consequently the needle will be more deflected than by the action of the jar alone. If the electricity of the body be negative, it will tend to neutralize the positive charge; and the needle will be less deflected. Hence the kind of electricity present in the air becomes at once apparent, without the necessity of an experimental test. The platinum wire must then be turned till the needle is brought to zero, and the number of degrees observed; which is a measure of the intensity of the electrification.

Any loss of charge from the Leyden-jar which may from time to time occur, reducing the sensibility inconveniently, may be made good by additions from a small electrophorus which accompanies the instrument.

The instrument may be made self-recording by the aid of clockwork and photography. To effect this, a clock gives motion to a cylinder, upon which photographic paper is mounted. The needle of the electrometer is made to carry a small reflector; and rays from a properly adjusted source of light are thrown by the reflector, through a small opening, upon the photographic paper. It is evident, that as the cylinder revolves, a trace will be left upon the paper, showing the magnitude of, and variations in, the deflection of the needle.