EXTENSION OF THE SIGNAL SERVICE. 509 



probably the most delicate and refined of any in the subject of elec- 

 tricity. Sir William Thomson's electrometer is an admirable instru- 

 ment, 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 coat- 

 ing 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-bat- 

 tery of five hundred or a thousand cells. Small glass vials, similar 

 to those used by the homoeopathists, 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 



