10 



The whole principle of the instrument depends upon the potential 

 of the needle remaining constant throughout the time a series of 

 observations is being made. The sensibility of the instrument 

 is really proportional to the potential of the needle ; therefore, by 

 altering the charge of the Leyden jar, we can alter at will the 

 sensibility of the electrometer. As this constant and normal 

 condition is very essential in making a series of measurements, 

 it is of great importance that some means be devised whereby 

 we can know at au}' time the electrical condition of the needle. 

 The latter is accomplished by the gauge connected with the instru- 

 ment. 



The Gauge (Fig. 3) is really a very delicate electrometer. It 

 consists of two metallic disks having their planes parallel and 

 close to each other. The upper disk G has a square hole imme- 

 diately over the centre of the lower disk. The latter is in electrical 

 connection with the acid of the jar. A piece of white sheet 

 aluminum, shaped like a spade, is carried on a tightly stretched 

 platinum wire, as seen in Fig. 3. The blade P covers the square 

 hole in the upper disk. The rung of the handle H is a very fine 

 black hair, and behind the hair stands a porcelain pillar with two 

 black dots upon it. The arrangement is looked at through a 

 plano-convex lens a little distance off. When the lower disk is 

 charged the blade is attracted and the rung of the handle is raised. 

 If the handle sinks below the mark, we know the potential of the 

 jar is falling. The instrument is in proper condition when the 

 hair is midway between the dots. The gauge is controlled by a 

 very delicate electrical machine called the " replenisher," turned 

 by the finger. 



Replenisher. — The replenisher consists of two curved shields, 

 one of which is in connection with the acid in the jar and the other 

 with the framework of the instrument, and through it the tin-foil 

 outside of the jar. Two metal wings, curving outward and insulated 

 from each other by a small bar of ebonite, are made to revolve 

 within the shields. Dui'iug such revolution of the wings, the latter 

 are made to come in contact with two springs connected together 

 but insulated from the rest of the instrument. The wings come 

 in contact with the two springs at the same time, and being thus 

 connected and under the influence of the shields, the positive 

 electricity of the left-hand shield, say, draws negative electricity 

 to the left-hand wing, close to it, and drives the positive to the 

 right-hand wing. 



Continuing the revolution still further, the wings clear the 



