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.[16] 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.