ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 249 



(3) General case, when electric charge resides on the surface of 

 the wire, The component of the electric field which is parallel 

 to the surface of a wire is always equal to the RI drop per 

 centimeter along the wire, but the component of the electric 

 field at right angles to the surface of the wire may have any 

 value whatever, and the electric lines of force which terminate on 

 the surface of the wire on account of the existence of this normal 

 component of electric field involve a stationary * electric charge 

 on the surface of the wire. An example of this general case is 

 shown in Fig. 180. An electric generator G delivers current 



over two line wires f A and B to a distant lamp L. The 

 electromotive force across from A to B involves the existence 

 of an electric field the lines of force of which trend somewhat as 

 shown by the full-line curves in the figure. The magnetic field 

 between A and B is everywhere perpendicular to the plane of 

 the paper and everywhere of the same intensity, so that the 

 energy stream lines are a series of lines which are everywhere at 

 right angles to the electric lines of force. The electric lines of 

 force where they touch A and B are slightly inclined to the 



* The electric current may be considered to be a transfer of electric charge 

 along the wire but the charge here referred to has nothing directly to do with the 

 current. When a voltaic cell is on open circuit, the electric field in the surrounding 

 region may be such that the volts per centimeter along a given path may vary in the 

 most irregular way ; but when this path is occupied by a wire through which the 

 voltaic cell produces a current, then the electric field is modified by the charge on 

 the surface of the wire so as to make the component of the electric field parallel to 

 the wire everywhere equal to the RI drop per centimeter along the wire. 



f In order that Fig. 1 80 may be a complete representation, A and B must be 

 supposed to be broad metal bands. 



