246 LECTURES TO SCIENCE TEACHERS. 



is usually done, we have the potential of the conductor 

 tested _ 



V'= (V - D) ' 



S 



Now it is plainly unnecessary to determine the absolute 

 distances D 7 and D. The difference, or the change of dis- 

 tance from a given position of the lower plate A, is all that 

 is required. This Sir William Thomson has found it easy 

 to measure with sufficient exactness by means of a micro- 

 meter screw. 



The form of absolute electrometer that I have now 

 described has been recently superseded by the instrument 

 shown (Fig. 10). The principles of the instrument are 

 precisely the same as those that I have been explaining. 

 The plates A and B, Fig. 10, correspond to those marked A 

 and B, Fig. 9. The plate c of the instrument just described 

 is, in the new instrument, hung somewhat differently, 

 being suspended on three springs somewhat like coach 

 springs. The movable plate and its suspending springs 

 are covered and protected from inductive influence under 

 a cylindrical cover D, of which a part is shown displaced in 

 the diagram. The plate A is moved up or down by a 

 micrometer screw moved by a milled head c' below. The 

 upper part of the glass cover is a Leyden jar, with which 

 is connected a gauge J, similar to that of the quadrant 

 electrometer, and R is a replenisher for maintaining the 

 Leyden jar at a constant potential. In this instrument, 

 however, it is the guard-ring B and the suspended disc that 

 are maintained at a constant potential ; while the conductor 

 to be tested is connected with the electrode E, which is 

 connected with the plate A by the spiral spring R. There 

 are other particulars of which I should like to speak to you, 

 had we the instrument itself before us, and did our time, 

 which I perceive is almost gone, permit. 



Lastly, I must show you a very beautiful instrument, the 

 Portable Electrometer (Fig. 1 1). A few words will suffice for 

 its description. In the bottom of a cylindrical glass jar, 

 of which a considerable part is coated with tin-foil, so as to 

 form a Leyden jar, there is fastened a circular brass plate, 

 with a movable aluminium square, and long index arm, pre- 

 cisely the same as the gauge of the quadrant electrometer 



