ELECTRICITY. 



441 



live E, and is continued to L, exactly above the centre of 

 Electricity. t }, e ball A. The length of this rod is six inches from 

 Y~^ F to L. At the extremity L is fixed a piece of amber 

 C, with an opening in the middle to admit the rod 

 DCB. The rod DB is formed of a silk thread coated 

 with sealing-wax ; the silk thread must be very strong 

 and very dry, anil, when completely penetrated by the 

 melted wax, it must be made straight and smooth, so as 

 to be sufficiently stiff and free of all asperities. This 

 rod passes through a small cube of amber, which has 

 fine holes drilled in two of its opposite sides ; and by 

 means of two round pins passing through the piece of 

 amber C, the rod DB has a free motion round these pins 

 as an axis. The branch CB of this rod is about three 

 inches long, and carries at its extremity a ball B a quar- 

 ter of an inch in diameter, which may be either made of 

 metal, or of cork gilt and burnished. The other branch 

 CD, which is of the same length, passes through a small 

 cork ball. When the instrument is thus constructed, the 

 lower ballB must just touch the ball A, when thearmFE 

 is in a vertical position. The ball F, Fig. 5. must now 

 be fixed at the extremity of a glass rod FI, which pa^s 

 at right angles through the centre of a divided circle 

 GHO, and has a handle of boxwood at its other ex- 

 tremity I. This rod, which is perpendicular to AF, 

 is supported by the head of a pillar II K, in which it 

 turns with some difficulty. An index NH, fastened 

 to the rod FI, is always set parallel to the line LA, 

 drawn through the -centre of the fixed ball. This in- 

 dex will point out the angle which LA forms with 

 the vertical line. It will also be convenient to have 

 another index, which turns stiffly on F I as an axis, 

 and stretches a considerable way beyond the gra- 

 duated circle. The graduated circle is divided into 

 360 degrees ; the zero of the division is placed upper- 

 most, and 90 on the right hand. 



cation In order to use this instrument, take hold of the 

 : in- handle I, and set the index to 90. The lines I. A andCB 

 ent to w iH now have a horizontal position, and the ball B will 



e the 



regt on 



j pt the |, a ]j s l)e nou . 



a ,,,l l t . t t |,e 



index be brought back to on the scale, by the handle I ; 

 during this motion the balls will be seen to separate in 

 some particular position of the index ; repeat this ope- 

 ration till the exact position is ascertained at which the 

 separation take* place. This will shew the repulsive 

 force when in contact, or at a distance equal to the 

 sum of the radii. Let the instrument be now turned 

 still more towards the vertical position, and the bails 

 will be observed to separate more and more. An as- 

 sistant must turn the long index, so as tc make it pa- 

 rallel, by making the one appear to coincide with the 

 other. In order to shew that this instrument will give 

 an absolute measure of the repulsiTe force of the two 

 balls, we have only to balance the rod BD in n hori- 

 zontal position by loading the ball D with some great- 

 er weight ; then, by computing for the proportional 

 lengths of BC and DC, we shall find the exact number 

 of grains with which the balls must repel each other, 

 in order merely to separate, when the rod BD has a 

 a horizontal position. When this rod Ivis any oblique 

 position, a very simple computation will give us the 

 number of grains of repulsion, with which they will 

 then separate ; and a third comjMitatien, founded on the 

 :tion (.['forces will give us the repulsive force in 

 grains, when A L has an oblique position, and when 

 the rod B 1) forms any angle. 



In order to make this instrument measure the.it- 

 ins of two balls, one of which is positively and 



VOL. VIII, PAHTir. 



the other negatively electrified, We must incline the Descriptive 

 instrument to the left, and by electrifying the balls l 

 A, B, in an opposite manner, when at a great dis- Application 

 tance from each other, they will approach by their of the in- 

 mutual attraction ; and by means of the deviation strument to 

 of BC from a vertical line towards A, we shall be able measure the 

 to determine the magnitude of the attractive force, j"^ at 

 In making experiments with this instrument, the great- 

 est care should be taken to provide as much as can be 

 done against any dissipation or waste of the electric 

 matter, by having all angles and asperities removed, by 

 varnishing all its parts, and by making the experiments 

 in a warm room, and when the atmosphere is particu- 

 larly dry. Dr Robison recommends, that the dissipa- 

 tion per minute, under various circumstances, should be 

 determined by a previous set of experiments. 



By meansof different instrumentsof this kind, of differ- General 

 ent sizes, and some of them of balls of an inch in diame- result. 

 tiT, and radii of 18 inches, Dr Robison performed more 

 than an hundred experiments, from which he concluded 

 that the mutual repulsion of two spheres, electrified in 



the same manner, varied as ,,. C6> d representing the 



distance of their centres. 



S'veral years after these experiments of Dr Robison's Coulomb's 

 were made, and read before a public society, theatten- experiments 

 tion of Coulomb was drawn to the same topic, by the " c !j^ tr i" 

 prize on the subject of the magnetic needle, which he atu . act i on . 

 had the good fortune to share with Van Swinden. His 

 first object was to contrive an apparatus for measuring 

 very small quantities of electricity. This apparatus, H' 9 torsl n 

 which he employed in all his experiments, and which he ' 

 calls a Ionian balance, is represented in Plate CCXLIV. PLATE 

 Fig. 7. where ABCD is a glass cylinder, 12 inches high f' x j' 1 ^ ' 

 and 12 inches broad, on which is placed a plateof glass lg ' 

 AC, 13 inches in diameter. This plate, which covers 

 the whole of the cylinder, is pierced with two hd\esf, m, 

 both ot which are 20 lines in diameter. Into the hole 

 yis cemented a tubeofglass/Vi twofeethigh, and at the 

 upper end of the tube at /< is placed a torsion micro- 

 meter, which is seen on an enlarged scale in Fig. 8. Fig. 8. 

 The upper part of this micrometer consists of a button 

 in, an index / u, and a pair of pincers if. This upper 

 part is let down into the hole (1, in the centre of the 

 circle a, b, which is divided upon its edge into !'6'0 de- 

 grees. This plate is soldered upon a tube of copper X, 

 which enters into the tube II, Fig. 9 ; and this tube is ce- Fig. 9. 

 mented into the upper extremity of the long glass tube 

 fh. The pincers q have nearly the shape of the extremity 

 of a solid port-crayon, which can be opened and shut 

 by means of ring r. These pincers lay hold of the 

 extremity of ;i very slender silver wire, the other end 

 of which is held at P, Fig. 10. by the pincers of an iron pj j 

 or copper cylinder P o, about a h'ne in diameter. These 

 pincers, like the former, are opened and shut by a ring Q. 

 This cylinder is perforated at C, to receive the needle 

 ag. The weight of this part of the apparatus must be suf- 

 ficiently great to stretch the silver wire without break- 

 ing it. The needle A g, which must be suspended ho- 

 rizontally about six inches from the bottom of the 

 cylinder ABCD, is formed of a silk thread, or a 

 covered with sealing-wax, and has the part <jn, alxnit 

 18 line? in length, formed of a cylinder of gum lac. 

 At t!ii extremity is a small ball a of the pith of elder, 

 about two or three lines in diameter ; and at the other 

 extremity g, is a small vertical plane of paper, covered 

 with turpentine, which serve* as a counterpoise to the 

 ball a. 



: K 



