ON STANDARDS OF ELECTRICAL RESISTANCE. 509 



extremely well, even in the smokj', dusty, and acidulated atmosphere of 

 Glasgow. When an extremely perfect insulation of the principal electrode 

 and connected attracting-plate is required, the vulcanite stopper surrounding- 

 it may be removed, so that the only communication between the electrode 

 and the case of the instrument may be along the two glass columns in the 

 artificially dried interior atmosphere of the case ; but from day to day, when 

 the instrument is out of use, the aperture round the principal electrode should 

 be kept carefully stopped, if not by a vulcanite insulator by a perforated cork 

 (although I find but little loss of insulation, either by the inner glass surface 

 of the Leyden jar or by the three glass columns, when this precaution is 

 neglected). 



(4) Temporary charging-rod supported by a vertical perforated column of 

 paraffined vulcanite, or a glass tube well varnished outside and thickly paraf- 

 fined inside. The insulating column bearing this charging-rod is turned 

 round till a horizontal spring projecting from its upper end touches the inner 

 coating of the jar, when this is to be charged from an independent source, 

 or when, for any other experimental reason, it is to be put in connexion with 

 a conductor outside the case of the instriuuent. 



(5) A small replenisher of the kind described for the quadrant electrometer 

 (§ 12), but with much wider air-sjiaces to prevent discharge by sparks. 



(6) A large glass or lead dish to hold as large masses of pumice as may be, 

 which are to be kept sufficiently impregnated with strong sulphuric acid. 



§ 36. A considerable position of the jar above the guard-plate is left im- 

 coated to allow the observer to see easily the hair and white background with 

 black dots ; also several other smaller parts of the glass above the guard-plate 

 are left uncoated to admit light to allow a small circiUar level on the upper 

 side of the guard-plate to be seen. The long arm of the aluminium balance- 

 lever is very thoroughly guarded by double cages and fences of wire (§ 25), 

 so that it can experience no sensible influence from electric disturbing forces 

 when the covering jar is put in position and electric connexion is established 

 between its inner coating and the guard-plate by projecting flexible wires or 

 slips of metal. 



§ 37. The aluminium square plate is somewhat larger, and the platinum 

 bearing wire somewhat longer in this instrument than in the portable electro- 

 meter, to render it sensible to smaller differences of potential. The step of 

 the screw is the same as in the portable (-^g of an iuch), and one division 

 (y^ of the circumference of the screw-head) corresponds to a diff'erence of 

 potentials which, roughly speaking, is equal to about that of a single cell of 

 DanieU's. The eff'ective range of the instrument is about sixty turns of the 

 screw, and therefore about 6000 cells of DanieU's. That of the portable 

 electrometer is about 15 turns of the screw (equivalent to about 5000 cells). 

 Neither of these instruments has sufficient range to measure the potential to 

 which Leyden jars are charged in ordinary electric experiments, or those 

 reached by the prime conductor of a powerful electric machine. The station- 

 ary instrument with its long screw and its large plates now described, would 

 go far towards meeting this want if its aluminium lever and platinum sus- 

 pension were made on the same scale as those of the portable electrometer ; 

 but for an instrument never wanted to directly measure differences of poten- 

 tials of less than two or three thousand cells, the heterostatic (§ 40) principle 

 is in general not iisefid, and therefore I have constructed the following very 

 simple idiostatic (§ 40) instrument, which is adapted to measure with con- 

 siderable accuracy differences of potential from 4000 cells upwards, to about 

 80,000 cells. 



