GENERAL ACCOUNT OF COMMON PHENOMENA 11 



these two charges come together and also neutralise each other 

 ami no electrification of either kind remains. 



When electrification is produced by friction the two electrifica- 

 tions are at first one on the rubber and the other on the rubbed 

 bodies. When the two bodies arc widely separated they may no 

 longer act only on each other, but on the walls. Thus a charged 

 ebonite rod which after being rubbed with wool is held up in the 

 middle of a room has its opposite + on the arm of the holder, on 

 the table, or on the walls. If two rods both negatively electrified are 

 held up near each other they appear to repel each other, but our 

 further study will show that we ought rather to regard them as 

 drawn apart by the opposite walls, which are positively electrified 

 by induction. 



Discharge of electrification. When an insulated 

 electrified body is touched by the finger or by a conductor 

 connected with the ground the electrification passes away and the 

 body is said to be discharged. We have described this discharge 

 as if the charge merely went to the earth. We can now see more 

 of the true nature of the process. The electrification of the body, 

 when insulated, is accompanied by opposite electrification of 

 the walls, floor, and surrounding conductors, and we may regard 

 the two charges as tending to come together, but unable to pass 

 through the separating insulating medium. When a conducting 

 bridge is made the two charges spread along or through it and 

 unite to neutralise each other. Frequently they are able to break 

 down the insulation of the medium if the electrification is con- 

 siderable, and a spark occurs. In all cases of electric discharge 

 the energy of the electrified system is dissipated in the discharge, 

 either as heat in the conductors, or as light, heat, and sound, and 

 perhaps chemical energy in the spark. 



Methods of producing electrification in large 

 quantity. Frictional machines. Many different forms of 

 machine have been devised for producing electrification con- 

 tinuously by friction, and for a long time they were in common 

 INC. As they are now superseded for most purposes by induction 

 machines, it will suffice if we describe one form Winter's plate 

 machine. A full account of the mode of constructing and using 

 tins and other frictional machines will be found in Harris's 

 Frictional Electricity. 



In Fig. lO/? is a circular glass plate mounted on a horizontal 

 axis, which can bi- turned by the handle h in the direction of the 

 arrow-head. The plate passes at the lowest point between two 

 pads of silk or leather, stuffed with wool, and these serve as 

 rubbers. Their power of exciting electrification is greatly 

 incmiM'd if an amalgam, usually of mercury, /inc, and tin, is mixed 

 with lard and smeared over the surfaces. The pads are backed 

 with wood and connected to earth, either through the woodwork of 

 tin- machine or, if this is not sufficient, directly by a wire to the 



