ELECTRICITY. 



t5 



ins; the globe or cylinder about its axis. 

 The principal use of such a coating is 

 to improve bad machines, for it is not 

 required in good ones. 



The earlier electricians contented 

 themselves with using; the hand as a 

 rubber, till a cushion was introduced 

 for that purpose by Professor Winkler, 

 of Leipsic. The cushion is usually 

 made of soft leather, generally basil 

 skin, stuffed with hair or wool, so as to 

 be as hard as the bottom of a chair, 

 but yet sufficiently yielding to accom- 

 modate itself, without much pressure, 

 to the surface of the glass to which it 

 is applied. 



(58.) Of cylindric machines, the sim- 

 plest and most perfect construction is 

 that invented by Nairne, and which is 

 represented in fig. 9. The glass cylinder 

 C is from 8 to i 6 inches in diameter, and 



Ft'g.9. 



from one to two feet long, supported, for 

 the purpose of insulation, on two up- 

 right pillars of glass, which are fixed to 

 a firm wooden stand. Two hollow me- 

 tallic conductors P, N , equal in length to 

 the cylinders, are placed parallel to it, 

 one on each side, upon two insulating 

 pillars of glass, which are cemented into 

 two separate pieces of wood that slide 

 across the base so as to allow of their 

 being brought within different distances 

 from the cylinder. To one of these 

 conductors N, the cushion is attached, 



being fastened to it by the intervention 

 of a bent spring, the purpose of which 

 is to keep it equally pressed against the 

 cylinder in every part of its revolution. 

 The pressure of the cushion is also fur- 

 ther regulated by an adjusting screw 

 adapted to the wooden base, on which 

 the glass pillar that supports the con- 

 ductor is fixed. From the upper edge 

 of the cushion there proceeds a flap P 

 of thin oiled silk, which is sewed on the 

 face of the cushion about a quarter of 

 an inch from its upper edge. It extends 

 over the upper surface of the glass 

 cylinder to within an inch of a row of 

 metallic points, proceeding like the teeth 

 of a rake from a horizontal rod, which 

 is fixed to the adjacent side of the oppo- 

 site conductor P. The motion of the 

 cylinder must- always be given in the 

 direction of the silk flap ; and it may 

 be communicated either by a single 

 handle, or by a multiplying wheel W, 

 as in the figure: the latter produces 

 more electricity in the same time, but 

 the labour of turning is increased nearly 

 in the same proportion. On some ac- 

 counts it is more convenient to place the 

 conductor to which the rubber is not 

 attached, at right angles to the cylinder ; 

 and this is the plan adopted in the com- 

 mon electrical machines. 



(59.) The conductor P, to which the 

 rubber is not attached, is generally 

 called the prime conductor, or the posi- 

 tive conductor, as the electricity with 

 which it becomes charged is positive. 

 It is a cylindrical tube, each end ter- 

 minating in a hemisphere. There is no 

 advantage in its being made of solid 

 materials, for the electricity is contained 

 only at the surfaces. It may be made 

 of thin sheet brass, or copper, or tin, or 

 of pasteboard, covered with gold leaf or 

 tin foil. Care must be taken that its 

 surface be free from all points and aspe- 

 i ities ; and the perforations which are 

 made in it, and which should be about 

 the size of a quill, for the purpose of 

 attaching wires, and other kinds of ap- 

 paratus, should have their edges well 

 rounded and smoothed off. For 'the 

 more perfect insulation of the conduc- 

 tor, it is advisable to apply upon the 

 glass pillar which supports it, a varnish 

 of gum-lac, or of sealing wax. 



(60.) The degree of excitation pro- 

 duced in the glass depends much upon 

 the substance employed as a rubber. 

 Mr. Singer observes that dry silk is 

 very efficacious, but that the most 

 powerful effects are obtained by the use 



