202 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



Fig. 127. 



to prevent the escape of the positive charge from the surface of 

 the disk.* 



111. Charging by influence. The simplest example of charg- 

 ing by influence is that which is described in connection with Fig. 



123. Charging by influence is 

 essentially the cutting of lines 

 of force in two by two sheets 

 of metal so that the ending of 

 the lines of force on one sheet 

 constitute a new negative 

 charge and the beginning of 

 the lines of force on the other 

 sheet constitute a new positive 

 charge. Let A, Fig. 127, be a 

 charged body from which lines 

 of force emanate. When a 

 metal ball B is brought near to 

 A, the lines of force converge 

 upon one side of B and diverge from the other side as shown in 

 the figure ; if a second metal ball C is brought into contact with 

 B, as shown in Fig. 128, then the lines of force converge upon 

 B and diverge from C, and the two balls B and C retain their 

 charges when they are separated and removed to a distance from 

 A. This operation is called charging by influence, and it results 

 in the production of equal amounts of positive and negative elec- 

 tricity (on B and C respectively) while the original influencing 

 charge on A is undiminished. Charging by influence is exem- 

 plified by the operation of the electrophorus. 



112. The electrophorus is a device for the production of charge 

 by influence. It consists of a rosin or hard rubber plate D, 



_*The frictional electric machine involves the principle of the electric doubler, but 

 it is not worth while to examine minutely into the manner in which the lines of force 

 are drawn out as it were and "strung" across from P to TV, as the various parts 

 of the glass plate leave 'the cushions A A. The above account, which is based on the 

 idea that positive and negative electricities are two fluid-like substances, is sufficiently 

 intelligible for present purposes. 



