378 



WSLLS'S NATURAL PHILOSOPHY. 



Describe the 

 two varieties 

 of electrical 

 machines in 

 commoa use. 



Electrical machines are of two kinds, the 

 plate and cylinder machines. They derive 

 their names from the shape of the glass em- 

 ployed to yield the electricity. 



Fig. 313. 



The plate electrical machine, which is 

 represented in Fig. 313, consists of a 

 large circular plato of glass mounted 

 upon a metallic axis, and supported up- 

 on pillars fixed to a secure base, so that 

 the plate can, by means of a handle, w, 

 be turned with ease. Upon the sup- 

 ports of the glass, and fixed so as to 

 press easily but uniformly on the plate, 

 are four rubbers, marked r r r r in the 

 figure ; and flaps of silk, s s, oiled on ono 

 side, are attached to these, and secured 

 to fixed supports by several silk cords. 

 When the machine is put in motion, 

 these flaps of silk are drawn tightly 

 against the glass, and thus the friction is 

 increased, and electricity excited. The 

 points p p collect the electricity from the glass as it revolves, and convey it to 

 the prime conductor, c, which is insulated and supported by the glass rod, g. 



The cylinder electrical machine represented by 

 Fig. 314, consists of a glass cylinder, so arranged 

 that it can be turned on its axis by a crank, and 

 supported by two uprights of wood, dried and 

 varnished. F S indicates the position and ar- 

 rangement of the rubber and silk, and Y that 

 of the prime conductor. The principle of the con- 

 struction of the cylinder machine is, in every 

 respect, the same as that of the plate machine. 

 What is the '^^® rubber of an electrical ma- 



chine consists of a cushion stuffed 

 with hair, and covered with 

 leather, or some substance which readily generates electricity by friction. 

 The efiBciency of the machine is greatly increased by covering the cushioa 

 with an amalgam, or mixture of mercury, tin, and zinc* 



In the ordinary working of the machine, the rubber is connected by a chain 

 with the ground, from whence the supply of electricity is derived. 



• The best composUion of the amalgam is two parts, by weight, of zinc, one of tin, and 

 Bis of mercury. The mercury is added to the mixture of the zinc and tin when in a fluid 

 state, and the whole is then shaken in a wooden box until it is cold ; it is then reduced to 

 a powder, and mixed with a sufficient quantity of lard to reduce it to the consistency of 

 paste. A thin coating of this paste is spread over the cushion ; but before this is done, all 

 darts of the machine should be carefully cleaned and warmed. 



construction of 

 the rubber ? 



