218 ELEMENTARY LESSONS ON [CHAP. iv. 



Leyden jar or other condenser. In order to observe 

 the minutest motions of the needle, a reading-telescope 

 and scale are so placed that the observer looking through 

 the telescope sees an image of the zero of the scale 

 reflected in the little mirror. The wires connecting 

 quadrants i and 3, 2 and 4, are seen above the top of 

 the case. The needle and quadrants are shown in plan 

 separately above. If there is the slightest difference of 

 potential between the pairs of quadrants, the needle, 

 which is held in its zero position by the elasticity of the 

 wire, will turn, and so indicate the difference of potential. 

 When these deflections are small, the scale readings will 

 be very nearly proportional to the difference of potential. 

 The instrument is sufficiently delicate to show a difference 

 of potential between the quadrants as small as the T V of 

 that of the Daniell's cell. 



For very exact measurements many additional refine- 

 ments are introduced into the instrument. Two sets of 

 quadrants are employed, an upper and a lower, having 

 the needle between them. The torsion wire is replaced 

 by a delicate bifilar suspension (Art. 1 1 8). To keep 

 up the charge of the Leyden jar a " Replenisher " is 

 added ; and an " attracted-disc," like that of the Absolute 

 Electrometer, is employed in order to act as a gauge to 

 indicate when the jar is charged to the right potential. 

 In these forms the jar consists of a glass vessel placed 

 below the quadrants, coated externally with strips of tin- 

 foil, and containing strong sulphuric acid which serves 

 the double function of keeping the apparatus dry by 

 absorbing the moisture and of acting as an internal 

 coating for the jar. It is also more usual to throw a 

 spot of light from a lamp upon a scale by means of the 

 little mirror (as described in the case of the Mirror 

 Galvanometer, in Art. 202), than to adopt the subjective 

 method with the telescope, which only one person at a 

 time can use. When the instrument is provided with 

 replenisher and gauge, the measurements can be made in 



