ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 269 



equal to the velocity of light as determined by direct observation. 

 Therefore the most accurate method for determining the value of 

 the constant B as used in Arts. 91 to 98 is to calculate its value 

 from the observed value of V using equation (8 1 ). 



The identity of the velocities of electromagnetic waves and of 

 light waves was first pointed out by Maxwell and it is now uni- 

 versally conceded that light waves are electromagnetic waves. 



147. Electric wave distortion. So long as the electric and 

 magnetic field intensities in the wave which is shown in Fig. 195 

 continue to satisfy equation (79), the electromagnetic wave remains 

 pure and it does not change its shape as it travels along. The 

 effect of the electrical resistance of the two bounding wires (or 

 metal sheets) is to cause a steady decay of the magnetic field, and 

 the effect of imperfect insulation of the material between the 

 bounding wires is to cause a continual decay of the electric field. 

 The continual decay of the magnetic field may be thought of as 

 due to the resistance which opposes the slipping of the rotating 

 ether cells where they are in contact with the bounding wires in 

 Fig. 195, and the continual decay of the electric field is somewhat 

 analogous to the slow disappearance of stress in a stretched piece 

 of rubber which may be supposed to have, in addition to its elastic 

 property, a certain degree of viscosity, like pitch, so as to con- 

 tinually yield under the influence of the stress. When the re- 

 sistance per unit length of the bounding wires in Fig. 195 bears 

 a certain ratio * to the insulation resistance of the material be- 

 tween unit length of the bounding wires, then the electric and 

 magnetic fields decay in such a way as to continually satisfy equa- 

 tion (79), and the wave progresses without changing its shape. A 

 pair of transmission wires which satisfies this condition constitutes 

 what is called a distortionless line. In all ordinary telephone lines 

 the effect of line resistance is greatly in excess of the effect of line 

 leakage, f and therefore an electric wave in being transmitted along 



* This relation may be quite easily formulated but an elaborate discussion of wave- 

 distortion is not within the scope of this text. 



t Several interesting examples are given by B. S. Cohen in The Electrician 

 (London), April 10, 1908. 



OF, ELECTRICAL LABORATORY,] 



