1868.] Mr. R. L. J. Ellery on a Pendulum-Electrograph. 461 



the top of the cylinder are two tubular apertures lined with plugs of ebo- 

 nite, through which the external parts of the electrodes pass. The key 

 for lifting the pendulum is also fitted on the top of the case. 



7. The charging-apparatus. — This consists of an ordinary electrophorus, 

 and of a rod of wire covered with gutta percha or ebonite, terminating at 

 one end in a brass knob, at the other in a projection of uncovered metal for 

 placing in contact with the ring which projects from the Leyden jar pillar. 



The air inside the cylinder is kept perfectly dry by two leaden trays, con- 

 taining lumps of pumice-stone saturated with sulphuric acid*. 



The arrangement of light, cylinder for photograph-paper and clockwork, 

 as also of the reading-telescope and scale, are in all respects similar to those 

 adopted for the vertical-force magnetograph of Kew, and described in the 

 Report of the British Association for 1859. 



The water-dropping apparatus is of the same kind as described by Sir W. 

 Thomson in Nichol's Cyclopaedia (1860, art. "Atmospheric Electricity''), 

 The cistern contains about twenty-eight gallons, which is found to give the 

 requisite stream for about thirty hours. It is a copper vessel, 2 feet square 

 and 1 foot high. Keeping it shallow avoids much alteration of head of 

 water, and consequently secures a more regular stream : this tank rests 

 upon four ebonite insulators indoors. A copper pipe ending in a fine nozzle 

 passes through a hole in the glass of the window, and projects to about four 

 feet beyond the wall of the building ; the tank is connected with the elec- 

 trometer in another chamber by means of a copper wire very thickly covered 

 with gutta percha. 



The mode of using the whole apparatus may be thus describod. The 

 Leyden jar or reservoir is first charged by a few good sparks from the elec- 

 trophorus ; to do this the cover of the opening (y) is removed and the 

 charging-rod inserted so that the bare end rests on the ring (c). The 

 sparks from the electrophorus are then passed to the brass knob, which 

 projects a few inches outside the case. The rod is then withdrawn and the 

 opening closed. The act of charging generally sets the pendulum oscilla- 

 ting ; it soon comes to rest, however. An hour after charging 1 have gene- 

 rally found the charge sufficiently permanent for commencing registration. 

 The two electrodes are then connected with the earth ; in a minute the 

 reading of the reflected scale will give what may be styled charge-zero, 

 which is always different from the zero-reading before charging the jar, 

 the latter being the reading of the position of rest of the pendulum (which, 

 by the bye, I make so sensitive as to vibrate about once a second). The 

 charge has now to be measured in terms of the slide and screw. To do 

 this, the wire is first pushed in till the disk is at some definite distance from 

 the pendulum-disk ; to admit of this being done precisely and quickly, a 



It is necessary that the sulphuric acid be quite pure, for nitric acid is often an im- 

 purity in commercial samples, which soon corrodes all the parts. To avoid this, it is 

 best to heat the sulphuric acid well in a platinum dish and drive off all the nitrous 

 fumes. 



