ON STANDARDS OP ELECTRICAL RESISTANCE. 495 



is limited by a properly placed stop. Another stop (e) above limits the upward 

 motion, which takes place imder the influence of elcctiification in the use of 

 the instrument. A. very fine opako black haii' (that of a small black-and-tan 

 terrier I have found much superior to any hitherto tried) is stretched across 

 the forked portion of the sheet aluminium in which the long arm of the lever 

 terminates. Looked at horizontally from the outside of the instrument it is 

 seen, as shown in fig. 7, Plate V., against a white background, marked with two 

 very fine black circles. These sight-plates in the instruments, as now made by 

 Mr. White, are of the same material as the ordinary enamel watch-dials, mth 

 black figiu'es on a white groimd. The white space between the two circles 

 should be a very Httle less than the breadth of the hair. The sight-plate is 

 set to be as near the hair as it can be without imjieding its motion in any 

 part of its range ; and it is slightly convex forwards, and is so placed that 

 the hair is nearer to it when in the middle between the black circles than 

 when in any other part of its range. It is thus made veiy easy, even with- 

 out optical aid, to avoid any considerable error of parallax in estimating the 

 position of the hair relatively to the two black circles. By a simple plano- 

 convex lens (f, fig. 2), with the convex side turned inwards, it is easy, in 

 the ordinary use of the instrument, to distinguish a motion up or down of 

 the hair amounting to g-^,; of an inch. With a little care I have ascertained, 

 Dr. Joule assisting, that a motion of no more than g^^^ of an inch from one 

 definite central position can be securely tested without the aid of other 

 magnifying-power than that given by the simple lens. The lens during use 

 is in a fixed position relatively to the framework bearing the needle, but it 

 may be drawn out or pushed in to suit the focus of each observer. To give 

 great magnification, it ought to be drawn out so far that the hair and sight- 

 plate behind may be but little nearer to the lens than its principal focus, 

 and the observer's eye ought to be at a very considerable distance from the 

 instrument, no less than 20 centimetres (8 inches) to get good magnification ; 

 and a short-sighted person shoidd use his ordinary concave eye-lens close to 

 Ms eye. The reason for turning the convexity of the small plano-convex lens 

 inwards is, that if the eye of the observer is too high or too low, the hair 

 seems to him curved upwards or downwards, and he is thus guided to keep his 

 eye on a level sufficiently constant to do away with aU sensible effects of parallax 

 on the position of the hair relatively to the black circles. The framework 

 carrying the stretched platinum wire and moveable attracted disk is above the 

 brass roof of the lantern, in which a square aperture is cut to allow the square 

 portion constituting the short arm of the aluminium balance to be attracted 

 downwards by the fixed attracting disk (§ 7), to be presently described. 

 A side view of the attracting plate, the brass roof of the lantern, the alu- 

 minium balance, the sight-plate, the hair, and the plano-convex lens is 

 shown in section (fig. 2), also a glass upper roof to protect the gauge and the 

 interior of the instrument below from dust and disturbance by currents of 

 air, to which, without this upper roof, it would be exposed, through the small 

 vacant space round tlic moveable aluminium square. The fixed attracting 

 disk is borne by a vertical screw screwing into the upper brass mounting 

 (z, fig. 2) (§ 7), connected with the inner coating of the Leyden jar, through 

 the guard tubes, &c., and is secured in any position by the " jam nut," shown 

 in the drawings at r, fig. 2. This disk (s) is circular, and about 38 millimetres 

 (1|- inch) diameter, and it is placed horizontally with its centre under the 

 centre of the square aperture in the roof of the lantern. Its distance 

 from the lower surface of the roof and of the moveable attracted disk may 

 be from 2jr to 5 millimetres (from J^- to ^ of an inch), and is to be adjusted, 



