ELECTRICITY (Suppl.) has the advantage; but in every other respect, Mr Coulomb's is the finest electrometer that has yet been published, giving absolute measures, and this with great accuracy. The Hon. Mr Cavendish has employed the construction in his most valuable experiments on the force of gravity (Phil. Trans. 1798, Part II.); an experiment which Newton would have been delighted with observing.
Culberson's ELECTROMETER is thus described by himself in the last number of the second volume of Nicholson's Philosophical Journal. GH (fig. 6.) is an oblong piece of wood, about 18 inches in length and six in breadth, in which are fixed three glass supporters, D, E, F, mounted with brass balls, . Of these supporters E and F are exactly of the same length; but D is four inches shorter. Under the brass ball is a long brass hook; the ball is made of two hemispheres, the under one being fixed to the brass mounting, and the upper turned with a groove to shut upon it, so that it can be taken off at pleasure. The ball has a brass tube fixed to it, about three inches long, cemented on the top of F, and the same ball has a hole at the top, of about one-half inch diameter, corresponding with the inside of the tube. AB is a straight brass wire, with a knife-edged centre in the middle, placed a little below the centre of gravity, and equally balanced with a hollow brass ball at each end, the centre, or axis, resting upon a proper shaped piece of brass fixed in the inside of the ball ; that side of the hemisphere towards is cut open, to permit the end of the balance to descend till it touches the ball , and the upper hemisphere C is also cut open to permit the end to ascend; is a weight, weighing a certain number of grains, and made in the form of a pin with a broad head; the ball B has two holes, one at the top, and the other at the bottom; the upper hole is so wide, as to let the head of the pin pass through it, but to stop at the under one, with its shank hanging freely in ; is a common Henly's quadrant electrometer; and when in use it is screwed upon the top of .
It is evident, from the construction, that if the foot stand horizontal, and the ball B be made to touch , it will remain in that position without the help of the weight ; and if it should by any means receive a very low charge of electric fluid, the two balls , will repel each other; B will begin to ascend, and, on account of the centre of gravity being above the centre of motion, the ascension will continue till A rest upon . If the balance be set again horizontal, and the pin be put into its place in B, it will cause B to rest upon , with a pressure equal to that weight, so that more electric fluid must be communicated than formerly before the balls will separate; and as the weight in B is increased or diminished, a greater or less quantity of electric fluid will be required to effect a separation.
When this instrument is to be applied to a jar, or battery, one end of a wire L must be inserted into a hole in , and the other end into a hole of any ball proceeding from the inside of a battery, as M. A chain, or wire, or any body through which the charge is to pass, must be hung to the hook at , and carried from thence to the outside of the battery, as is represented by the line N. must be screwed upon , with its index towards A. The reason of this instrument being added, is to shew, by the index continuing to rise, that the charge of the battery is increasing, because the other
from other part of the instrument does not act till the battery has received its required charge.
It is almost needless to observe that this instrument consists of three electrometers, viz. Henley's electrometer, Lane's discharging electrometer considerably improved, and Brookes's leeward electrometer improved likewise. By this combination and these improvements, we possess all that can be required in an electrometer for batteries and large jars; for, by , we see the progress of the charge; by the separation of & , we have the repulsive power in weight; and by the ball , the discharge is caused when the charge has acquired the strength proposed.
In the journal from which this abstract is taken, the reader will find some curious experiments made with batteries by means of this electrometer; but one will be sufficient to explain its use. Prepare the electrometer in the manner shewn in the figure, with the jar annexed, which contains about 168 square inches of coating. Take out the pin in , and observe whether the ball will remain at rest upon ; if not, turn the adjusting screw at till it just remains upon . Put into the pin, marked , weighing 15 grains; take two inches of watch-pendulum wire, fix to each end a pair of spring tongs, as is represented at , hook one end to , and the other to the wire , communicating with the outside of the jar; let the uncoated part of the jar be made very clean and dry; and let the prime conductor of an electrical machine, or a wire proceeding from it, touch the wire ; then, if the machine be put in motion, the jar and electrometer will charge, as will be seen by the rising of the index of ; and when charged high enough, will be repelled by , and will descend and discharge the jar through the wire which was confined in the tongs, and the wire will be fused and run into balls. The ingenious author, by breathing through a glass pipe into the jar, dumped it a little in the inside. Then loading with a pin of 30 grains, he obtained such a charge as fused eight inches of watch-pendulum wire, disposed exactly as the two inches were disposed in the former experiment. By repeating and varying his experiments, he found that double quantities of electrical fluid, in the form of a discharge, will melt four times the length of wire of a certain diameter.