Page:Popular Science Monthly Volume 20.djvu/525

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probably the most delicate and refined of any in the subject of electricity. Sir William Thomson's electrometer is an admirable instrument, wonderful in its details and in its scientific plan. It can be used, however, by any intelligent observer. In principle it consists merely of a suspended aluminum needle which swings between four hollow quadrants. The needle is put in constant contact with the inner coating—which consists of sulphuric acid—of a Leyden-jar, by means of a small wire suspended from the needle. The diagonal quadrants are connected. If one pair is put in connection with the ground and the other pair connected with the body whose electrical state is to be tested, the aluminum needle by its deflection indicates the kind and amount of the electrical state. The needle is provided with a mirror, which allows its deflections to be observed by means of the reflection of a point of light. Although the principle is simple, yet the means for obtaining complete insulation and for keeping a constant charge in the needle are quite complicated. The Leyden-jar can be charged by means of a small Holtz machine, or by means of an induction-coil. A still better method of charging has been adopted by the Continental physicists: this consists in the employment of a miniature water-battery of five hundred or a thousand cells. Small glass vials, similar to those used by the homœopathists, are filled with distilled water, are placed in a wooden box, and are well insulated from each other. A sheet of zinc and copper are soldered together along their edges, and then cut into small couples of zinc and copper. These cells are coupled seriatim, or, as it is usually termed, for intensity. The entire battery need not occupy a space of more than a foot and a half square, and can be made even more compact. The statical charge at the terminals of this miniature battery is strong, and a Leyden-jar or a condenser can be easily charged by connecting one terminal of the battery to the inner coating of the condenser and the other to the outside coating. The electrometer-jar is charged, therefore, in this way, by being kept permanently connected with this battery.

The apparatus for collecting the charge of the air, in order to affect the charged needle, consists of an insulated tin can filled with water. The water is allowed to flow from this can by means of an horizontal glass tube, which is drawn out to a comparatively narrow orifice. The can is then connected by an insulated wire to one pair of the quadrants of the electrometer. Since the original charge of the can and the water—all objects are necessarily charged with electricity to a greater or less extent—is dissipated by the flow of the water, the electricity of the air continually renews this loss, and the tin can finally takes the charge of the air in which it is placed. This method is very sensitive, and will detect discharges of lightning by a throb of the electrometer-needle even when the thunder-storm has not yet appeared above the horizon. A method of obtaining continuous registrations of the deflections of the electrometer-needle is necessary. This can