ON STANDARDS OF ELECTRICAL RESISTANCE. 493 



jar, the outside of the jar being coated with tinfoil and connected with the 

 earth, so that it is in reality a Leyden jar. The whole outside of the jar, 

 even where not coated with tinfoil, is in the ordinary use of the instrument, 

 especially in our moist climate, kept virtually at one potential through con- 

 duction along its surface. This potential is generally, by connecting wires 

 or metal pieces, kept the same as that of the brass legs and framework of 

 the instrument. To prevent disturbance in case of strongly electrified bodies 

 being brought into the neighbourhood of the instrument, a wire is eitlier 

 wrapped round the jar from top to bottom, or a cage or network of wire, or any 

 convenient metal case, is placed round it ; but this ought to be easily removed 

 or opened at any time to admit of the interior being seen. When the instru- 

 ment is left to itself from day to day in ordinary use, the needle, connected 

 with the inner coating of the jar as just described, loses, of course, imless 

 replenished, something of its charge ; but not in general more than | per 

 cent, per day, when the jar is of flint glass made in Glasgow. On trying 

 similar jars of green glass I found that they lost their charge more rapidly 

 per hour than the white glass jars per month. I have occasionally, but very 

 rarely, found white glass jars to be as defective as those green ones, and it is 

 possible that the defect I found in the green jars was an accident to the jars 

 tested, and not an essential property of that kind of glass. 



§ 12. I have recently made the very useful addition of a replenisher to 

 restore electricity to the jar from time to time when required. It consists of 

 (1) a turning vertical shaft of vulcanite bearing two metal pieces called carriers 

 (b, b, figs. 17 & 18) ; (2) two springs {d, d, figs. 16 & IS, Plate V.), connected 

 by a metaUic arc, making contact on the carriers once every half turn of the 

 shaft, and therefore called connectors ; and (3) two inductors (a, a) with re- 

 ceiving springs (c, c) attached to them, which make contact on the carriers 

 once every half turn, shortly before the connecting contacts are made. The 

 inductors {a a, figs. IG & IS) are pieces of sheet metal bent into circular cylin- 

 drical shapes of about 120° each ; they are placed so as to deviate in the man- 

 ner shown in the di-awing from parts of a cylindrical surface coaxal with the 

 turning-shaft, leaving gaps of about G0° on each side. The diameter of this 

 cylindrical surface is about 15 millimetres (about | an inch). The carriers 

 (6 b, figs. 17 ife IS) are also of sheet metal bent to cylindrical surfaces, but not 

 exactly circular cylinders ; and are so placed on the bearing vulcanite shaft 

 that each is rubbed by the contact springs over a very short space, about 1 

 millimetre beyond its foremost edge, when turned in the proper direction for 

 replenishing. The receiving springs (c, c, figs. 17 & IS) make their contacts 

 with each carrier immediately after it has got fairly under cover, as it were, 

 of the inductor. Each carrier subtends an angle of about 60° at the axis 

 of the turning-shaft. The connecting contacts are completed just before the 

 carriers commence emerging from being under cover of the inductors. The 

 carriers may be said to be under cover of the inductors when they are within 

 an angle of 120° on each side of the axis subtended by the inductors. One 

 of the inductors is in metallic communication with the outside coating of the 

 jar, the other with the inside. Figs. 16, 17, & 18 illustrate sufficiently 

 the shape of carriers and the succession of the contacts. The arrow-head 

 indicates the direction to turn for replenishing. When it is desired to dimi- 

 nish the charge, the replenisher is tiu'ned backwards. A small charge having 

 been given to the jar from an independent source, the replenisher when 

 turned forwards increases the difference of potentials between tlie two 

 inductors and the two coatings of the jar connected with them by a constant 

 percentage rper half turn, unless it is raised to so high a degree as to break 



