ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 255 



surface of a wire which carries an electric current is charged,* 

 and this charge is associated with the lines of force of electric 

 field which is perpendicular to the surface of the wire. There is 

 always a point on any wire circuit, however, where the wire is 

 not charged or, in other words, where the electric field is parallel 

 to the surface of the wire. In any case, however, it is permissible 

 to consider that part of the energy flow which depends upon the 

 component of the electric field parallel to the wire. Figure 198 

 shows a straight wire carrying electric current. The lines of 

 force of the electric field are parallel to the -wire as shown in the 

 side view, and the lines of force of the magnetic field encircle the 

 wire as shown in the end view. The dotted lines represent the 

 energy stream which flows in towards the wire from all sides. 



The equation for the energy stream in an electromagnetic 

 field may be established by considering the inward flow of energy 

 in the neighborhood of a wire carrying an electric current as 

 follows: Let R be the resistance of the wire in abohms per 



* The component of the electric field which is parallel to the surface of a wire 

 is always equal to RI, where R is the resistance of the wire per centimeter of 

 length, and / is the current flowing in the wire; but the component of the electric 

 field at right angles to the surface of the wire may have any value whatever. The 

 electric lines of force which terminate on the surface of the wire on account of 

 the existence of this normal component of the electric field involve a stationary 

 electric charge on the surface of the wire. The electric current is of course con- 

 sidered to be a transfer of electric charge along a wire, but the stationary 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 current, 

 then the electric field in the whole surrounding region is modified by the stationary 

 charge on the surface of the wire so as to make the component of the electric field 

 parallel to the wire everywhere equal to RI, as above specified. Energy appears hi 

 each unit length of the wire at the rate of RI 2 ergs per second. This amount of 

 energy must flow into every unit length of the wire, and the electric field in the 

 neighborhood of the wire must be so distributed as to give this necessary distribu- 

 tion of the energy stream. It is to be remembered that the trend of the magnetic 

 field in the neighborhood of an electric circuit depends only on the shape of the 

 circuit but not at all on the relative resistances of the various parts of the circuit, 

 and, therefore, the proper distribution of the energy stream to supply the RI 2 losses 

 at each part of a circuit depends, one might say, chiefly upon the modification of 

 the electric field due to surface charges on the wire. 



