278 The Philosophical Society of Glasgow. 



the portable apparatus shown in the third drawing. In it the 

 index is attached at right angles to the naiddle of a fine pla- 

 tinum wire, firmly stretched between the inside coatings of two 

 Leyden phials, and consists simply of a very light bar of aluminium, 

 extending equally on the two sides of the supporting wire. It is repelled 

 by two short bars of metal, fixed on the two sides of the top of a metal 

 tube, which is supported by the inside coating of the lower phial, and 

 has the fine wire in its axis. A conductor of suitable shape (Fig. 3), bearing 

 an electrode, to connect with the body to be tested, insulated inside the 

 case of the instrument, in the neighbourhood of the index, and when 

 electrified in the same way, or the contraiy way, to the inside coatings 

 of the Leyden phials, causes, by its influence, the repulsion between the 

 index and the fixed bars to be diminished or increased. The upper 

 Leyden phial is moveable about a fixed axis, through angles measured 

 by a pointer and circle, and thus the amount of torsion, in one-half of 

 the bearing wire, required to bring the index to a constant position in 

 any case is measured. The square root of the number of degrees of 

 torsion measures the difference of potentials between the conductor 

 tested and the inner coating of the Leyden phial. In using the instru- 

 ment, the conductor tested is first put in connection with the earth, 

 and the torsion required to bring the index to its fixed position is read 

 off. This is called the zero, or earth reading. The tested conductor is 

 then electrified, and the torsion reading taken. In the atmospheric 

 application, this is called the air reading. The excess — positive or 

 negative — of its square root, above that of the zero reading, measures 

 the electro-motive force between the earth and the point of air tested. 

 This result, when positive shows vitreous, when negative resinous 

 potential in the air, if the index is resinous. By the aid of Barlow's 

 table of square roots, the indications of the instrument m&j thus be 

 reduced to definite measure of potential, almost as quickly as they can 

 be written down. Once for all, the sensibility of the instrument can be 

 determined by comparison with an absolute electrometer, or a galvanic 

 battery. In the portable apparatus a burning match is used — instead 

 of the water-dropping system, which the writer finds more convenient 

 than any other for a fixed apparatus — to reduce the insulated conductor 

 to the same potential as the air at its end. It is in reality the electri- 

 fication of the earth's surface which has either directly or virtually been 

 the subject of measurement in all observations on atmospheric electricity 

 hitherto made. Tlie methods which have been followed may be divided 

 into two classes — (1.) Those in which means are taken to reduce the 

 potential of an insulated conductor to the same as that of the air, at 

 some point a few feet or yards distant from the earth. (2.) Those in 



