CHAP, i.] ELECTRICITY AND MAGNETISM. 43 



ball or knob. The general aspect of the machine is 

 shown in Fig. 23. When the handle is turned the 

 friction between the glass and the amalgam -coated 

 surface of the rubber produces a copious electrical 

 separation, electricity being produced in the rubber, 

 + electricity being carried round upon the surface of the 

 glass. The prime conductor collects this electricity by 

 the following process : The + charge on the glass acts 

 inductively on the long insulated conductor, repelling + 

 electricity to the far end and attracting to the nearer 

 end. The effect of the row of points is to let off the 

 attracted electricity in a continuous discharge directed 

 towards the attracting + charge upon the glass, which is 

 neutralised thereby, the glass thus arriving at the rubber 

 in a neutral condition ready to be again excited. This 

 action of the points is sometimes described, though less 

 correctly, by saying that the points collect the + 

 electricity from the glass. If it is desired to collect also 

 the electricity of the rubber, the cushion must be 

 supported on an insulating stem and provided at the 

 back with a metallic knob. This device, which thus 

 enables either electricity to be used at will, is due to 

 Nairne. It is, however, more usual to use only the + 

 electricity, and to connect the rubber by a chain to 

 " earth," so allowing the charge to escape. 



4O. The Plate Electrical Machine. The Plate 

 Machine, as its name implies, is constructed with a 

 circular plate of glass or of ebonite, and is usually pro- 

 vided with two pairs of rubbers formed of double 

 cushions, pressing the plate between them, placed at its 

 highest and lowest point, and provided with silk flaps, 

 each extending over a quadrant of the circle. The prime 

 conductor is either double or curved round to meet the 

 plate at the two ends of its horizontal diameter, and is 

 furnished with two sets of spikes, for the same purpose 

 as the row of points in the cylinder machine. A 

 common form of plate machine is shown in Fig. 24. 



