E L E T li M E T E It 121 connected with the guard plate to avoid all electrical forces on the hiilnnce ; the hair of the balance is brought to the sighted position, thus ascertain how far the weight of w grammes depresses the balance. The weight is now removed, and the balance left at a distance above A equal to that just found. A is now charged, and its potential adjusted till the hair of the gauge in dicates that the standard potential v is reached. Let it now be required to measure the difference between the poten tials V and V of two conductors. Con nect first one and then the other with n, and work the lower screw till the hair of the balance is sighted in each case, and let the screw readings reduced to centi metres be d nnd d . Then, since the force on the disc in each case is gw, where g is the acceleration produced by gravity in a falling body in centimetres per second, we have by (1) (5), Fig. 14. and the upper is distributed again by worl FIG. 13. Dry Pile Electroscope. screw reading taken ; then a weight of w grammes symmetrically OR the disc, the balance brought up ung the screw, and the reading again taken. We where S denotes the area of the balance disc, or rather the mean of the areas of the disc and the hole in which it works. We thus get the value of V - V in absolute electrostatic C. G. S. units. III. Symmetrical Electrometers. Two instruments fall to be Dry ] ilo described under this head, the dry pile electroscope, and Thomson s electro- quadrant electrometer. The idea common to these instruments scope. is to measure differences of potential by means of the motions of an electrified body in a symmetrical field of force. In the dry pile electroscope, a single gold leaf is hung up in the field of force, between the opposite poles of two dry piles, or, in later forms of the instrument, of the same dry pile. The original inventor of this apparatus was Behrens, but it often bears the name of Bobnen- berger, who slightly modified its form. Feehner introduced the important improvement of using only one pile, which he removed from the immediate neighbourhood of the suspended leaf. The poles of the pile are connected with two discs of metal, between which the leaf hangs. This arrangement makes it easier to secure perfect symmetry in the electric field, and allows us to vary the sensitiveness of the instrument by placing the metal plates at different distances from the leaf. In order to make the attainment Fro. 15. Elevation and Section of Thomson s Quadrant Electrometer. of perfect symmetry still more easy and certain Riess 1 added a metal rod to the apparatus, which can be made to touch the two metal caps of the dry pile simultaneously, and then lie re moved, leaving the pile symmetrically electrified. This form of the electroscope, with the various improvements, is represented in lig. 18. 1 Reibungselfctr., 16. Hankel 2 still further improved the dry pile electroscope by giving a micrometric movement to the plates, by substituting a galvanic battery with a large number of cells for the uncertain and varying dry pile, and by using a microscope with a divided scale to measure the motions of the gold leaf. With these improvements it became an Electrometer of great delicacy and considerable range. Some of the
- Mascart, 272, or Poyy. Ann., 1858.
