CHAP, iv.] ELECTRICITY AND MAGNETISM. 219 



terms of absolute units, provided the " constant " of the 

 particular instrument (depending on the suspension of 

 the needle, size and position of needle and quadrants, 

 potential of the gauge, etc.) is once ascertained. 



263. An example will illustrate the mode of using the instru- 

 ment. It is known that when the two ends of a thin wire are 

 kept at two different potentials a current flows through the wire, 

 and that if the potential is measured at different points along 

 the wire, it is found to fall off in a perfectly uniform manner 

 from the end that is at a high potential down to that at the low 

 potential. At a point one quarter along the potential will have 

 fallen off one quarter of the whole difference. This could be 

 proved by joining the two ends of the wire through which the 

 current was flowing to the terminals of the Quadrant Electro- 

 meter, when one pair of quadrants would be at the high 

 potential and the other at the low potential. The needle would 

 turn and indicate a certain deflection. Now, disconnect one of 

 the pairs of quadrants from the low potential end of the wire, 

 and place them in communication with a point one quarter 

 along the wire from the high potential end. The needle will 

 at once indicate that the difference of potential is but one quarter 

 of what it was before. 



Often the Quadrant Electrometer is employed simply as a 

 very delicate electroscope in systems of measurement in which a 

 difference of electric potential is measured by being balanced 

 against an equal and opposite difference of potential, exact 

 balance being indicated by there being no deflection of the 

 Electrometer needle. Such methods of experimenting are known 

 as " Null Methods," or "Zero Methods." 



264. Dry-Pile Electrometer. The principle of 

 symmetry observed in the Quadrant Electrometer was 

 previously employed in the Electroscope of Bohnenberger 

 a much less accurate instrument in which the charge 

 to be examined .was imparted to a single gold leaf, placed 

 symmetrically between the poles of a dry-pile (Art. 182), 

 toward one or other pole of which the leaf was attracted. 

 Fechner modified the instrument by connecting the + 

 pole of the dry-pile with a gold leaf hanging between 

 two metal discs, from the more + of which it was re- 



