ELECTRICITY. 



495 



die*. 



DescHpfire nected with the positive end, and A with the negative 



Electricity. en( i ( the motion of the wheel will be from B to A. The 



"*" ""V'' transmission of a small charge through the wires, by 



an insulated jar, will produce the same effect. 



The preceding experiment, imagined by Mr Singer, 

 is considered by him as a proof that there is only one 

 electric fluid, and that it passes from the positive to the 

 negative wire ; for if there were two electric fluids, he 

 concludes " that the wheel, being equally acted upon 

 by each, will obey neither, and remain stationary." 

 We do not pretend to explain the motion of the wheel 

 from the positive to the negative wire ; but we cannot 

 help thinking, that the great question respecting the 

 existence of one or two fluids, must be decided by ex- 

 periments of a very different kind. If it could be 

 shewn that the result of the preceding experiment is 

 explicable only upon the supposition of there being one 

 electric fluid, we should have less hesitation in adopt- 

 ing Mr Singer's conclusion ; but the phenomenon may 

 arise from some other cause with which we are still 

 unacquainted. 



Perforation Exp. 4. Place a piece of card against the outside 

 of non-con- coating of a Leyden phial, and bring the lower knob 

 ducting bo- of the discharging rod towards the outside coating, so 

 that the thickness of the card is interposed between it 

 and the tinfoil ; then, when the jar is discharged by 

 bringing the upper knob in contact with the knob of 

 the jar, the charge will pass through the card and per- 

 forate it. On the side of the card, next the dischar- 

 ging rod, there will be a small bur or protrusion, and 

 on the other side a larger bur. The holes in the card 

 will be larger or smaller, according to the humidity or 

 dryness of the card. A quire of paper, or the board of 

 a book, may also be perforated by a great charge, and 

 if the paper be freely suspended, it will be perforated 

 without receiving the least motion, in the same manner 

 as a ball will pass through an open door without turn- 

 ing it on its hinges. When this experiment was made 

 by Mr Symmer, the paper was laid in a horizontal po- 

 sition, and he found that the ragged etlges pointed 

 mostly outwards from the body of the quire. When 

 he turned over the leaves, however, he found that the 

 edges of the holes were bent regularly two different 

 ways, and more remarkably so, about the middle of the 

 quire, one part of each hole upwards, and the other part 

 downwards, so that tracing any particular hole as it 

 traversed the quire, he fourtd that on one side the fi- 

 bres pointed one way, and on the other side the other 

 way, as if the hole had been made in the paper, by 

 drawing through it two thread-, in opposite direc- 

 tions. * 



Mr Gough has lately shewn, t that when the pre- 

 ceding experiment is accurately performed, the bur on 

 the negative side is always the largest. 



Exp. 5. Place in the middle of a paper book, about 

 the thickness of a quire, a slip of tinfoil, and, in another 

 paper book of the same thickness, place two other 

 slips of the same kind of tinfoil, separated by the two 

 middle leaves of the book. Let a strong charge be now 

 made to pass through these two books in succession, 

 and tlie following effects will be visible. In the first 

 book, the paper leaves on each side of the tinfoil will 

 be perforated, while the tinfoil itself is merely in- 

 dented in opposite directions, and the burs and the per- 

 foration will point different ways. In the second book, 

 all the leaves of paper will be penetrated, excepting 



the two that are included between the slips of tinfoil, 

 and in these two leaves there will be two impressions 

 or indentations, in opposite directions. 



Mr Symmer, who made the experiment in the pre- 

 sence of Dr Franklin, concluded from it, that there 

 were two opposite currents of electricity arising from 

 electric fluids, and the same conclusion has been drawn 

 by Mr Ezekiel Walker. Mr Singer, however, has 

 endeavoured to shew, that these effects are produced 

 solely by an expansion of the paper. 



Exp. 6. Take a small phial, or a glass tube sealed her- 

 metically at one end, and fill it nearly with olive oil. 

 Having put a cork into the phial or tube, introduce a 

 wire through it, so that its sharp point may touch the 

 inside of the phial or tube beneath the surface of the 

 oil. Let the vessel be now suspended to the prime 

 conductor of an electrifying machine, and the machine 

 being worked, bring either the knuckle or a brass ball 

 exactly opposite to the point of the wire within, so that 

 a sp;irk may pass between the wire and the knuckle, 

 the passage of the electricity through the glass will 

 perforate it completely. By bringing the wire in con- 

 tact with different parts of the glass, a great number of 

 holes may be made in it. 



Exp. 7. Place the ends of the two wires at the dis- 

 tance of an inch from each other, so that the end of one 

 of them is above a card, and the other below it ; then 

 when the discharge is made between the wires, a lumi- 

 nous flash will pass from the positive to the negative 

 \vire, along the surface of the card, and a hole will be 

 purforaU'd opposite to the point of the negative wire. 

 This happens even when the card has been perforated 

 before hand, opposite to the positive conductor. This 

 experiment is by M. Lullin of Geneva. 



Kcp. 8. Make the preceding experiment under the 

 receiver of an air pump, and, in proportion as the air 

 is exhausted, the place of perforation will happen near- 

 er the positive wire. When the air is half exhausted. 

 the hole is precisely half way between the two wires. 

 At every discharge, a flash passes from each conductor 

 to the place of perforation. This experiment was con- 

 trived by M. Tremery. 



Exp. 9. Into a piece of soft tobacco-pipe clay, or 

 any other kind which is neither too dry nor too moist, 

 introduce two wires, so that their distance within the 

 clay is not very great. Let a shock be passed through 

 the wire, as already described, and it will be found that 

 the clay is curiously expanded in the interval between 

 the wires. If the clay is too dry, or the shock too 

 strong, it will be shivered into innumerable fragments. 

 If the clay is placed in the tube of a tobacco pipe, or in 

 a glass tube, and if the shock is sent through it as be- 

 fore, the expansion of the clay will be so powerful as to 

 shatter the tube which contains it. 



In the course of his experiments on the effects of ex- 

 plosion through metallic substances, Dr Priestley obser- 

 ved, that a chain through which he had sent the charge 

 of a battery was shorter than it was before. He then 

 measured two feet four inches of the chain when it lay 

 upon the table, and having sent through it a charge of 

 (j-t square feet of coated glass, it was shortened a quar- 

 ter of an inch in its whole length. 



Mr Nairne repeated the experiment with a piece of 

 hard drawn iron wire 10 inches long, and -rj^th of an 

 inch in diameter. He discharged 26 feet of coated 

 glass nine times through the wire ; after the sixth and 



Descriptive 

 Electricity. 



Expansive 

 cfticts of 

 electricity. 



Exhibited 

 in the short- 

 ening of 



See Pkii Trant. 1759, vol. li. p. 371. 

 f Nicholson's Journal, vol. xxxii. p. 176. 



