526 



ELECTRIC IT Y. 



separated from B. In this situation, two of the plates 

 1 have evidently nearly equal quaiit .- ;e kind of 



electricity, while the other plate has the opposite elec- 

 tricity. The plate A is then applied to touch the un- 

 der surface of C, which remains upon the electro" 

 and C is at the same time covered with B. The plate 

 B is then touched, by stretching a finger over the junc- 

 ture of its insulating handle, ami removing A, and then 

 withdrawing the finger from B, and lifting it up from 

 C, the electricity becomes doubled. 



If the electricity is not sensible after duplication, Mr 

 Bennet proposes to repeat the operation 10 or 20 times, 

 which, by doubling it every time, will render visible 

 the smallest coin-enable quantity of electricity, for at 

 the end of the 20th operation it is augmented above 

 500,000 times. This operation, so often repeated, may 

 appear tedious, yet when the art of performing it rea- 

 dily is acquired, it will not occupy 10 seconds. When 

 it is required to elicit sparks, the plates must be placed 

 upon an insulating stand, without an electrometer, and 

 the doubling repeated till the sparks are emitted. 



In making the preceding experiments, great care 

 must betaken not to excite any electricity by the fric- 

 tion of the finger upon the varnished side of the plate. 

 In order to avoid this, Mr Bennet joined to each of the 

 plates a conducting handle of unhakcd mahogany, by 

 means of an insulating nut of baked wood, covered 

 with sealing wax, so that he did not require to touch 

 the sealing wax of the insulated nut, but occasionally 

 to stretch a finger over it, to touch the plate, whilst the 

 mahogany handle was held in the hand. Mr Bennet 

 likewise put thimbles on the ends of the touching 

 fingers ; but after every precaution, he still found that 

 electricity was produced in the instrument without 

 previous communication. Sec Phil. Trans. 1787, p. 

 288. 



SECT. II. Darwin's Movcable Doubler. 



Is order to avoid the labour of the preceding ope- 

 ration, Dr Darwin in vented the mo veable doubler, con- 

 sisting of four metallic plates, which could be moved 

 by wheel work into positions which required them to 

 lie touched by the hanc in order to produce the effect. 

 This instrument was shewn to Mr Nicholson in De- 

 cember 1787, and it occurred to him that the whole of 

 the operation might be performed by the rotation of a 

 simple winch, and hence he was led to the construction 

 of the following instrument. 



SECT. III. Nicholson' 't Revolving Doubler. 



Tins instrument is represented in Figs. 1. and 2. 

 icvulving of Plate CCL. It is supported on a glass pillar (>^ 

 rfoublrr. inches long, and consists of two fixed plates of bras* 

 PLATK A,-C, two inches in diameter, separately insulated, and 

 CCL. placed in the same plane, so that a revolving plate B 



'' * may pass near them without touching. A brass ball D, 

 two inches in diameter, is fixed on the end of the axis 

 that carries the plate B, and is loaded within at one 

 side, to act as a counterpoise to the revolving plate B, so 

 as to keep it at rest in any position The axis PN is 

 made of varnished glas, and so are the axes that join 

 the three plates with the brass axis NO. The axis NO 

 passes through the brass piece M, which supports the 

 plates A ana C. At one extremity of this axis is the 

 ball D, and the other is connected with a rod of glass 

 M', upon which the handle L is fixed, and also the 

 piece GH separately insulated. The pins E. F rise out 



Dvwin'i 



of the back of the fixed plates A, C, it unequal dis- 

 tances from the axis. The pieie K, is parallel to (.11; 

 and both of them have their ends armed with small ^ ... , 

 pieces of harpsichord wire, that they may touch the pin> doublet. 

 I'., V in certain points of their revolution. In the piece 

 M there is fixed a pin I, winch intercepts a small wire f,'^" 

 proceeding from the rev oh ing plate B. These wires iigs.1,5. 

 are so adjusted by bending, that when the plate B is 

 exactly opposite to I), tin- piece (HI connects the two 

 fixed plates, while the wire and pin at 1 fi/rm a comu e- 

 tion between the ball D and the plate 1!. On the con- 

 trary, when the plate B is exactly opjw.site to the plate 

 C, the ball D becomes connected with ( , by the con- 

 tact of F, with the wire at K, the plates A and B lx,'ing 

 then completely unconnected with any other part of the 

 apparatus. In all other positions, the three plates and 

 the ball will be perfectly unconnected with each oilier. 



" When the plates A and B," says Mr Nicholson, " arc Op''oi> 

 opposite to each other, the two fixed plates A and C' may "t the in- 

 be considered as one mass j and the revolving plate B, 6m " 

 together with the ball D, will constitute another mass. 

 All the cxpcrimcnU yet made, concur to prove that these 

 two masses will not possets the same electric state ; but 

 that, with respect to each other, their electricities will be 

 plus and minus. These plates would be simple, and with- 

 out any compensation, if the masses were remote from 

 each other ; but as that is not the case, a part of the re- 

 dundant electricity will take the form of a charge in the 

 opposed plates A and B. From other experiments. I 

 find that the effect of the compensation on plates oppo- 

 sed to each other at the distance of one fortieth part of 

 an inch, is such, that they require to produce a given 

 intensity at least 100 times the quantity of electricity 

 that would have produced it in cither singly and apart. 

 The redundant electricities in the masses under consi- 

 deration, will therefore be unequally distributed : tile 

 plate A will have about 99 parts, and the plate C one : 

 and, for the same reason, the revolving plate B will 

 have 99 parts of the opposite electricity, and the ball 

 D one. The rotation, by destroying the contacts, pre- 

 serves this unequal distribution, and carries B from A 

 to C, at the same time that the tail K connects the ball 

 with the plate C. In this situation, the electricity in 

 B acts upon that in C, and produces the contrary state, 

 by virtue of the communication l>etween C and the ball ; 

 which last must therefore acquire an electricity of the 

 same kind with that of the revolving plate. But the 

 rotation again destroys the contact, and restores B to 

 its first situation opposite A. Here, if we attend to 

 the effect of the whole revolution, we shall find that 

 the electric states of the respective masses have been 

 greatly increased ; for the 99 parts in A and in B re- 

 main, and the one part of electricity in C has been in- 

 creased so as nearly to compensate 99 parts of the op- 

 posite electricity in the revolving plate B, while the 

 communication produced an equal mutation in the elec- 

 tricity of the ball. A .second rotation will of course 

 produce a proportional augmentation of these increased 

 quantities, and a continuance of turning will soon bring 

 the intensities to their maximum, which is limited by 

 an explosion between the plates. 



If one of the parts be connected with an electrome- 

 ter, more especially that of Bennet, these tf. t< will be 

 very clearly seen. The spark is usually produced by 

 a number of turns between 11 and '20; a:id the elec- 

 trometer is sensibly acted upon by still fewer. \Vliun 

 one of the parts is occasionally connceted with the 

 earth, or when the adjustment of the pl.itci i.s altered, 

 there are some variations in the effects not difficult to 



