16 



ELECTRICITY. 



of an amalgam of tin, zinc, and mer- 

 cury, applied by means of hog's lard, to 

 the surface of leather or oiled silk. That 

 part of the cushion which comes in con- 

 tact with the glass cylinder, should be 

 coated with an amalgam of this kind, 

 spread evenly over its surface, until 

 level with the line formed by the seam 

 which joins the silk flap to the face of 

 the cushion. No amalgam should be 

 placed over this seam, nor on the silk 

 flap ; which last should be wiped clean 

 whenever the continued motion of the 

 machine shall have soiled it, by deposit- 

 ing dust or amalgam on its surface. The 

 same attention is requisite to the sur- 

 face of the glass, which often becomes 

 covered with black spots and lines, 

 more particularly when the amalgam 

 has been recently applied. It is essen- 

 tial to remove these as often as they 

 are formed in any quantity, since they 

 tend to lessen the power of the machine. 

 The surface of the amalgamated cushion 

 is also soon soiled; for the excited 

 glass constantly attracts dust from sur- 

 rounding bodies, and this dust is col- 

 lected by the rubber as the glass passes 

 it. If the dust is removed after every 

 course of experiments, by separating 

 the cushion from the negative conductor, 

 and gently rubbing its surface, and the 

 surface of the silk flap, with a dry 

 linen cloth, the machine may be kept in 

 good order without a frequent renewal 

 of the amalgam ; such renewal being 

 only necessary when that which has 

 been applied becomes irregularly dis- 

 tributed over the cushion, or impreg- 

 nated with dust. 



(61.) The amalgam recommended by 

 Mr. Singer, is made by melting together 

 one ounce of tin and two ounces of 

 zinc, which are to be mixed, while 

 fluid, with six ounces of mercury, and 

 agitated in an iron, or thick wooden 

 box, till cold. It is then to be reduced 

 to very flne powder in a mortar, and 

 mixed with a sufficient quantity of hog's 

 lard to form it into a paste. When amal- 

 gams have a large proportion of mer- 

 cury, their action is variable and tran- 

 sient. The best cement for attaching 

 the cylinder to its pivots, is made by 

 mixing five pounds of resin, one pound 

 of bees' wax, one pound of red ochre, 

 and two table- spoonfuls of plaster of 

 Paris. The ochre and plaster of Paris 

 should be well dried, and then added to, 

 and alternately mixed with the other 

 ingredients, when they are in a state of 

 fusion. 



The plate machine, fig. 1 0, was ori- 

 ginally proposed by Dr. Ingenhouz, and 

 has been since much improved by Cuth- 

 bertson. This machine, in its most per- 



Fig. 1 0. 



feet form, consists of a circular plate of 

 glass, turning on an axis that passes at 

 right angles through its centre : it is 

 rubbed by two pair ^of cushions, fixed at 

 opposite parts of the circumference by 

 elastic frames of thin mahogany, which 

 are constructed so as to press the glass 

 plate between them with the requisite 

 force, by means of regulating screws. 

 A brass conductor P, supported by 

 glass, is fixed to the frame of the ma- 

 chine, with its branched extremities 

 opposite to each other, and near the 

 extreme diameter of the plate, in a direc- 

 tion at right angles to the vertical line 

 of the opposite cushions. The branched 

 extremities of the conductor are fur- 

 nished with pointed wires, that serve to 

 collect the electricity from the surface 

 of the excited plate. 



(62.) It is not quite determined which 

 of these two arrangements affords the 

 greatest quantity of electricity from the 

 same surface ; but the cylinder is less 

 expensive, and less liable to accidents 

 than the plate, and it appears to possess 

 nearly equal power. 



(63.) From what has already been 

 explained of the general laws of elec- 

 tricity, the mode in which these ma- 

 chines act will readily be understood. 

 The friction of the cushion against the 

 glass cylinder produces a transfer of 

 electric fluid from the former to the 

 latter; that is, the cushion becomes 

 negatively, and the glass positively, elec- 

 trified. The fluid which thus adheres 

 to the glass, is carried round by the re- 

 volution of the cylinder ; and its escape 

 is at first prevented by the silk flap 



