VIII. MEASUREMENT. 287 



present forms is given in Wiillner's Experimental Physics, Vol. 4, 3rd edition, 

 p. 159, and p. 299. 



1431. Edelmann's Quadrant-electrometer. 



M. Th. Edelmann, Munich. 



1431a. Electrometer for measuring potentials, and par- 

 ticularly the potential of an accumulator at the moment of the 

 discharge. P r f> Augustus Righi, Bologna. 



This electrometer consists of a brass cylindrical box (5), whose inferior 

 base breaks a small circular hole, while the superior is reduced, but at two 

 opposed sectors of 60. It is carried by two columns of ebonite fixed upon 

 a wood table supplied with level screws. A vertical brass tube rises on the 

 centre of the superior face of the box, and breaks the movable system (3), 

 namely, a double sector of aluminium, which in the apparatus is in part 

 covered by the double sector of the box, suspended by a platina wire, and 

 connected with a plane mirror and a little thin glass. This glass plunges in 

 some liquid, and serves to stop the oscillation. A brass cylinder (6) is fixed 

 under the box, and serves to protect the mirror from the influence of the 

 electricity, which can be transmitted in ebonite columns. The apparatus 

 reposes on a table (7), the upper part of which is covered with zinc, and 

 supported by three insulators ; a zinc bell (2), 570 millimeter height, must be 

 placed upon the insulated table (7), to contain the whole apparatus. A 

 metallic wire, covered by an ebonite tube (1), serves to establish the commu- 

 nication between the apparatus and one of the surfaces of accumulation (which 

 can always communicate with the electric machine which serves to charge it) ; 

 the other surface communicates with the zinc bell. It is necessary to charge 

 the condenser slowly so that the movable system may deviate without oscil- 

 lation ; at the moment of the discharge the effected deflection is read as usual, 

 by the reflection of a mirror on a scale. This deflection is proportional to the 

 square of the difference between the electric potential of the apparatus and 

 that of the zinc bell ; otherwise the square root of the deflection measures the 

 difference of potential between the two surfaces of the condenser. 



For regulating the sensibility, a brass disc (4) can be put at different heights 

 along the brass tube. If this disc is brought down so as to shut the box, the 

 mirror does not deviate, not even with the strongest charges. 



By turning the small ball at the top of the apparatus, the movable system 

 is lowered or raised. If the brass piece which is under the small ball (in 

 Fig. 3) be turned instead, the aluminium double sector is displaced horizontally. 

 Finally by turning the upper part of the box, the mirror is turned towards the 

 hole in the cylinder 6. 



143 Ib. An Induction Electrometer. 



Prof. Augustus Rig hi, Bologna. 



A caoutchouc tube carrying many copper rings is wrapped round the non- 

 insulated pulleys (2, 4). If the insulated inductor (1) is charged, the rings 

 go from 2 to conductor 3, with charges of contrary sign, and these charges 

 remain in the insulated conductor 3. For, as the rings touch the conductor 

 3 by means of a little pulley placed in its interior, the charge preserved by 

 any single ring is neutralized. Briefly, for a very little charge of the 

 inductor, the conductor 3 acquires a sufficient charge, that can be shown by 

 a gold leaf electroscope. If the inductor is uncharged, and the pulleys and 



