212 ELEMENTARY LESSONS ON [CHAP, iv 



sign of a charge of electricity ; some of these also served 

 to indicate roughly the amount of these charges, but none 

 of them save the torsion balance could be regarded as 

 affording an accurate means of measuring either the 

 quantity or the potential of a given charge. An instru- 

 ment for measuring differences of electrostatic potential is 

 termed an Electrometer. Such instruments can also 

 be used to measure electric quantity indirectly, for the 

 quantity of a charge can be ascertained by measuring 

 the potential to which it can raise a conductor of known 

 capacity. The earliest electrometers attempted to measure 

 the quantities directly. Lane and Snow Harris constructed 

 " Unit Jars " or small Leyden jars, which, when it was 

 desired to measure out a certain quantity of electricity, 

 were charged and discharged a certain number of times. 

 The discharging gold-leaf electroscope of Gaugain was 

 invented with a similar idea. 



26O. Repulsion Electrometers. The torsion 

 balance, described in Art. 15, measures quantities by 

 measuring the forces exerted by the charges given to the 

 fixed and movable balls. It can only be applied to the 

 measurement of repelling forces, for the. equilibrium is 

 unstable in the case of a force of attraction. 



There are, besides the gold-leaf electroscope and the 

 Lane's electroscope, described in Lesson II., a number 

 of finer electrometers based upon the principle of repul- 

 sion, some of which resemble the torsion balance in 

 having a movable arm turning about a central axis. 

 Amongst these are the electrometers of Dellmann and of 

 Peltier ; the latter of these is shown in Fig. 1 1 1, in the 

 Lesson on Atmospheric Electricity. In this apparatus a 

 light arm of aluminium, balanced upon a point, carries 

 also a small magnet to direct it in the magnetic meridian. 

 A fixed arm, in metallic contact with the movable one, 

 also lies in the magnetic meridian. A charge imparted 

 to this instrument produces a repulsion between the fixed 

 and movable arms, causing an angular deviation. Here, 



