xlii DESCRIPTION OF PLATES. 



Fig. 559. A plate machine. A and B, the rubbers, which are usually double ; 

 C D, double flaps of oiled silk, for confining the electricity ; E, the conductor. 

 P. 525. 



Fig. 560. An electrophorus. A, the cake of resin ; B, the plate of metal ; C, 

 the ball for taking the spark ; D, the handle of glass. P. 526. 



Fig. 561 . A condenser, as arranged by Mr. Cavallo, under the name of a collector : 

 the middle plate is insulated : the two outward plates communicate with the earth ; 

 they stand near the first plate when the electricity is imparted to it, and are after- 

 wards removed by means of their hinges. P. 526. 



Fig. 562. Mr. Cavallo's multiplier. ' The electricity being first communicated to 

 the insulated plate A, the moveable plate B is brought near it, while the wire C 

 touches the pin D so as to form a communication with the earth ; the plate B is 

 then made to communicate with E, which is insulated, and stands near the plate F, 

 which enables it to receive almost the whole of the electricity brought at each alter- 

 nation by B ; and when the plate F is removed from the neighbourhood of E, this 

 plate becomes strongly charged. P. 527. 



Fig. 563. A revolving doubler, on the principle of Mr. Bennet's instrument. 

 The fixed and insulated plate A first receives the electricity, and when the moveable 

 plate B stands opposite to it, it receives by a wire from the stand of the instrument C 

 the opposite electricity ; when it is brought opposite to D, this plate is made to com- 

 municate with the stand by the wire E, and acquires a charge similar and nearly 

 equal to that of A. When B comes again to A, the wire F forming a communica- 

 tion between A and D, nearly the whole charge of both these plates is brought into 

 A, and B receives a charge almost twice as great as at first. P. 527. 



Fig. 564. Mr. Coulomb's electrical balance. The needle A is made of silk, 

 covered with sealing wax ; it supports, at the end B, a ball of the pith of elder, 

 another similar ball being fixed at C ; the force of attraction or repulsion is ascer- 

 tained by the torsion of the wire A D, which is measured by a graduated circle E. 

 P. 528. 



Fig. 565. Mr. Henry's quadrant electrometer; it is made of box wood, sup- 

 ported by metal : the ball is of cork, the graduated arc of ivory. P. 528. 



Fig. 566. A, Mr. Bennet's gold leaf electrometer ; B, a piece of excited sealing 

 wax held over it, for distinguishing the electricity. Instead of the pieces of gold leaf 

 C, we may substitute Mr. Cavallo's pith balls D, or the straws E, employed by Volta. 

 P. 528. 



Fig. 567. Mr. Lane's discharging electrometer. The distance of the balls A, B 

 is measured by the turns of the screw on the scale C ; and the parts of a turn are 

 ascertained by the graduated circle D. P. 528. 



Fig. 568. A discharger for a battery. When the repulsion of the balls A, B, 

 becomes greater than the weight of a wire which passes through a perforation in the 

 balls, they separate, and the ball C, descending to D, forms a communication, which 

 completes the circuit, so that the shock passes through any substance placed at E. 

 P. 528. 



PLATE XLI. 



Fig. 569. The form of the curves which show the direction of the magnetic 

 needle, in consequence of the attraction and repulsion of two poles, situated at A and 

 B. They are found by drawing the lines A CD, BED, so that the sum or dif- 

 ference of the parts A C, B E, shall be always equal, A C E B being a semicircle ; 

 and the direction D F may be found by making A F to B F as the cube of A D to 

 that of B D. P. 534. 



Fig. 570. The arrangement of iron filings in the neighbourhood of a magnet. 

 P. 534. 



Fig. 571. The particle of iron A B, lying on a card nearly over the magnet C, 

 assumes, when the card is shaken, first the position D, then, falling to E and F,fe 

 left a little further from the magnet than at first. P. 534. 



Fig. 572. An azimuth compass. The box is turned round, until the shadow of 

 the thread A B or A C falls on the line CD: the position of the needle is theft 



