ELECT R M E T E R graduated disc PL, through the centre of which passes a glass tube F supporting NE, so that it can be raised or depressed by a lever G. Inside F is a spring by means of which the lever H, which serves as electrode, can be connected or disconnected at will with the metal piece NE. The whole contained in a metal case B, the lid of which is of glass, so that the position of the needle D on the graduation PL can be read off by means of the lens M. To use the instrument, the case is connected with the earth, the needle is brought nearly at right angles to NE, and NE is raised by 119 FIG. 7. Bellman s Electrometer. means of G till the needle is in contact with it ; then the electrode K is brought into communication with NE, and the body whose charge or potential is to be measured is connected with K. The connection with K is then suppressed, and NE loweied; and the needle, now free, is repelled by NE. It, by means of the torsion head, we bring the needle along to a fixed position relative to NE, the electrical couple will be proportional to the square of the chaise communicated to NE and D, i.e., to the square of the potential of the body connected with K, provided the capacity of the electro meter be negligible compared with that of the body. Hence the potential is measured by the square root of the torsion on the fibre when the needle is in a given position. The form of Dellmann s electrometer we have just described was that used by Kohlrausch. 1 It has been simplified by its inventor, and applied in his important investigations on atmospheric elec tricity. Coulomb s balance might be used as an electrometer on the repul sion principle. Special care would, however, be necessary to avoid or to allow for disturbances arising from the case of the instrument, which ought under any circumstances to be coated wholly or par tially with tinfoil on the inside, according to Faraday s plan, Sir Win. Thomson did, in f.ict, design an electrometer of this descrip tion, and has given tables (Reprint of Papers, 142) for reducing its indications. Phis type of electrometer has not come into general use. II. Attracted Disc Electrometers, The first idea of this kind of FIG. 8. Snow Harris s Disc Electrometer instrument is due to Sir Wm. Snow Harris. One of the instru ments in which he carried out the principle and the mode of using 1 Pogrt. Ann., 1842 and 1852. it will be understood from fig. 8, C is an insulated disc, over which is suspended another disc, hung from the arm of a balance and connected with the earth. A weight w is put in a scale attached to the other arm of the balance. The insulated disc is connected with the internal armature B of a Leyden jar whose outer armature is in connection with the suspended disc Elec tricity is conveyed to B, and the quantity q measured by a small Lanes jar A, until the electric attraction at C is just sufficient to turn the balance Snow Harris found that WK q This and other laws established by him agree with the mathematical theory as developed in the article ELFcrmciTY. 2 Great improvements have been effected in this kind of electro meter by Sir Wm Thomson-(l) by his invention of the "guard ring or guard p ate ; (2) by using the torsion of a platinum wi.e for the standard force; (3) by devising proper means for attaining a definite standard potential, and by protecting thn vital parts of the electrometer from extraneous disturbance ; and (4) bv introducing sound kinematical principles into the construction of tne movable parts. In order to illustrate these points it will be well to describe the Thorn. portable electrometer (fig. 9), one of his simpler instruments, in son s de S!f l . portable Ihe principal electrical parts of this electrometer are sketched in electro- H is a plane disc of metal (called the "guard plate") meter, kept at a constant po tential by being fixed to the inner coating of a small Leyden jar MM (fig. 9), which forms) the case of the instru ment. At F a square hole is cut out of HH, and into this fits, as nearly as it can without danger of touching, a square piece of alumi nium foil as light as is consistent with proper stiffness. One .side of this disc is bent down, and then runs out hori zontally into a narrow stem ending in a stirrup L the whole being not unlike a spade. The sole of the stirrup con sists of a fine hair, which moves up and down be fore a vertical enamelled piece bestridden by the fork of the stirrup." On the enamel are two small dots very near each other. When the hair seen through a small convex lens appears straight, and bi sects the distance, between the dots, the stirrup is said to be in the sighted position. The alumi nium spade is suspended on a hori zontal platinum wire stretched by platinum springs at its two ends, and is carefully balanced with its centre of gravity in the line of suspension, so that the only force other than electric that can affect it is the torsion of the wiie, which acts like the string in the toy called the "jumping frog," ci like the hair rope in the catapultaof theancients. The spade is so arranged that F is as nearly as possible in the same plane with the guard plate when the hair is in the sighted position. It is (he torsional couple exerted by the wire in this position that forms the standard force. The remaining important electrical part is the plane horizontal disc G. It is essential to the action of the instrument that we should be able to move the disc G parallel to itself and to HI! through measured distances. The mechanism bv which this is accomplished is a remarkable instance of the application of geometrical principles to mechanism, and the reader will do well to read Thomson s " Lesson to the instrument makers" on this subject in the Rcjrrint of his papers, 369. The glass stem which carries G is fixed into the lower end of a hollow brass cylinder ; in the upper end of the cylinder is fixed a nut AC, through which works a carefully cut screw ending in a rounded point B of polished steel. The point B rests on a horizontal agate plate let into a foot which projects from 2 See also Reprint of Sir Wm. Thomson s Papers, 158. FIG. 9 Section of Thomson s Portable Electrometer.
Fig 10.