276 The Philosophical Society of Glasgoio. 



index of metal supported by a glass fibre, or a wire, stretched in the 

 line of the axis of the ring, and capable of having its fixed end turned 

 through angles measured by a cu-cle and pointer. (3.) A Leyden phial, 

 with its insulated coating electrically connected with the index. (4.) 

 A case to protect the index from currents of air, and to keep an artifi- 

 cially di-ied atmosphere round the insulating supports — glazed to allow 

 the index to be seen from without, but with the inner surface of the 

 glass screened (electrically) by wire cloth, perforated metal, or tinfoil, 

 to do away with irregular reflections on the index. In the instrument 

 represented in the drawing (Fig. 2), the ring is divided into only two 

 parts, which are equal, and separated by a space of air about one- 

 twentieth of an inch. Each of these half rings is supported on two glass 

 pillars ; and by means of screws acting on afoot which bears these pillars, 

 it is adjusted and fixed in its proper position. The index is of thin sheet 

 aluminium, and projects in only one direction from the glass fibre bear- 

 ing it. A stiff vertical wire, rigidly connected with it, nearly in the 

 prolongation of the fibre, bears a counterpoise considerably below the 

 level of the index, and heavy enough to keep the index horizontal. A 

 thin platinum wire, hooked to the lower end of this vertical wire, dips 

 in sulphuric acid in the bottom of the Leyden phial. The Lej'den 

 phial is charged either positively or negatively ; and is found to retain 

 its charge for months, losing, however, gradually, at some slow rate, 

 less generally than one per cent, per day of its amount. The index is 

 thus, when the instrument is in use, kept in a state of charge corre- 

 sponding to the potential of the inside coating of the phial. Wheii one 

 of the half rings is connected with the earth, and a charge of electricity 

 communicated to the other, the uidex moves from or towards the latter, 

 according as the charge communicated to it is of the same or the 

 opposite kind to that of the index. This instrument, as an electro- 

 scope, possesses extreme sensibility — much greater than that of any 

 other hitherto constructed ; and by the aid of the torsion arrangement, 

 it may be made to give accurate metrical results. There are some 

 difficulties in the use of it, especially as regards the comparison of the 

 indications obtained with different degrees of electrification of the index, 

 and the reduction of the results to absohite measure, hitherto obviated 

 only by a daily application of Delmann's method of reference to a zinc- 

 copper water battery, which Delmann himself applies once for all, to 

 one of his electrometers (unless his glass fibre breaks, when he must 

 make a fresh determination of the sensibility of the instrument with its 

 new fibre). The high sensibility of the divided ring electrometer 

 renders this test really very easy, as not moi'e than from ten to twenty 

 cells ai-c required ; and a comparison with a few good cells of Daniell's 



