510 REPORT— 1867. 



LoNG-EANGE Electrometer. 



§ 38. In this (fig. 15, Plate VI.) the continuous attracting-plate is above, 

 and the guard-plate with aluminium balance below, as in the portable elec- 

 trometer ; but, as in the standard stationary electrometer, the upper plate is 

 fixed and the lower plate is moved up and down by a micrometer-screw. The 

 mechanism of the screw and slide has aU the simpUcity and consequent accu- 

 racy of that of the standard electrometer. In the only long-range instrument 

 yet constructed the step of the screw is the same as that of the others (^ of 

 an inch). In future instruments it would be well either to have a longer step 

 or to have a simple mechanism (which can bo easily added) to give a quick 

 motion ; as in the use of the present instrument, the turning of the screw re- 

 quired for great changes of the potential measured is very tedious. The guard- 

 plate projects by more than an inch all round beyond the rim of the upper 

 attracting-plate ; partly to obviate the necessity of giving it a thick rim, which 

 would be required to prevent brushes and sparks originating in it, if it had 

 only the same diameter as the continuous plate above, and partly to guard 

 the observer from rcceiying a spark or shocli in measuring the potential of 

 an electric machine or of a Leyden battery, and to prevent the hair from being 

 attracted to the upper plate. Thus the guard-j^late is allowed to be no thicker 

 than sufiices for stiffness, and this allows the observer to see the hair at the 

 end of the aluminium balance-lever without the lever being made of a dy- 

 namically disadvantageous shape, as would be necessary if the guard-plate 

 were thick or had a thick rim added to it. No glass case is required for this 

 instrument. The smaUness of the needle and the greatness of the electric 

 force acting on it are such that I find in practice no disturbance to any in- 

 convenient degree by ordinary currents of air; although it and all these 

 attracted disk instruments show the influence of sudden change of baro- 

 metric pressure, such as that produced by opening or shutting a door. If 

 not kept under a glass shade when out of use, the lower surface of the upper 

 attracting-plate and the lower siu-face of the guard-plate and attracted alu- 

 minium square should be carefully dusted by a dry cool hand. Generally 

 speaking, none of the vital electric organs of an electrometer should be 

 touched by a cloth, as this is almost sure to leave shreds fatal to their healthy 

 action. 



§ 39. The effective range of this instrument is about 200 tiirns of the screw; 

 rather greater force of torsion is given than in the portable electrometer, and 

 a rather smaller attracted disk may be used, so that upwards of four cells 

 maybe the electric value of one division. The instrument in its present state 

 measures nearly but not quite the highest potential I can ordinarily produce 

 in the conductor of a good Winter's electric machine, which sometimes gives 

 sparks and brushes a foot long. 



§ 40. The classification of electrometers given above is founded on the shape 

 and kinematic relations of their chief organic parts ; but it will be remarked 

 that another principle of classification is presented by the different electric 

 systems used in them, which may be divided into two classes : — 



I. Idiostatic, that in which the whole electric force depends on the electri- 

 fication which is itself the si;bject of the test. 



II. Heterostatic, in which, besides the electrification to be tested, another 

 electrification maintained independently of it is taken advantage of. 



Thus, for example, the long-range electrometer (§§ 38, 39) is simply 

 idiostatic and is not adapted for heterostatic use ; but each of them may be 

 used idiostatically. The absolute electrometer was at first simply idiostatic 



