462 Mr. R. L. J. Ellery on a Pendulum-Eledrograph, [June 18, 



small stop {z) is fixed on the rim, and when the stop is home and the screw 

 and sHde reading two inches, the disks will be exactly two inches apart. 

 The disk is then gradually approached by means of the screw until, by the 

 attraction exerted upon the pendulum and consequent movement of the 

 mirror, the reading of the scale has altered any definite but small amount 

 (say, =10') ; the indices of the slide and screw are then noted and entered 

 as the charge-reading. The photographic cylinder is now adjusted to its 

 place, and the dot of light falling on it ascertained to be of its proper bright- 

 ness ; for after charging and after great atmospheric disturbances the pen- 

 dulum will be found to have shifted, slightly displacing the dot laterally, 

 and rendering it weak and undefined ; a gentle lifting and lowering of the 

 pendulum, however, by means of the lifting-apparatus and screw sets this 

 right immediately. 



The air-electrode {p) is now disconnected from the earth, but p' is left 

 connected ; the needle then assumes a slightly different position, which after 

 five minutes is read off by the telescope and entered as the earth-reading. 

 This position becomes also photographed, and will appear as a short but 

 undisturbed line on the sheet. At the end of the five minutes the wire 

 leading to the water-dropper is attached and the whole left for twenty-four 

 hours ; at the expiration of this period the tank is detached, and five mi- 

 nutes after the earth-reading taken and entered as before. The two elec- 

 trodes are then connected and the charge measured, the light and barrel 

 readjusted for the second day's curve, scale-reading observed and entered 

 again, and so on for the next twenty-four hours. 



The curves show at the commencement and end of each day a short 

 straight line already referred to, and corresponding to the mirror's 

 position for the earth-reading; a line drawn from one to the other of 

 these gives the zero or base-line for the day ; any loss in the charge of 

 the jar, which is assumed to be proportional to the time, is thus accounted 

 for ; for the line will be inclined to the edge of the sheet in proportion to 

 the loss in the twenty-four hours. In the case of this electrometer, the 

 charge of the jar being positive, and the front electrode being the one con- 

 nected with the air, a positive charge will raise the reflected beam of light, 

 and a negative one lower it ; so that the curve above this zero-line indicates 

 positive, and below it negative electricity. 



It now remains to show how the indications of the apparatus are 

 reduced. The standard I have adopted is one cell of Daniell's battery. 

 The deflections of the pendulum will be very nearly a joint measure of the 

 charge of the jar and of the electric potential of the air where the water 

 breaks into drops ; the charge of the jar, as I have shown, is arbi- 

 trarily measured in terms of the screw every day ; and the value of these 

 measures, in terms of Daniell's cells, is obtained as follows : — A 12-cell 

 Daniell's battery is placed at hand : the reservoir is first charged pretty 

 highly and left for an hour ; the electrodes are then both connected with 

 the earth, and the reading of the reflected scale obtained as a zero. The 



