ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 249 



fiM for the sake of brevity. Imagine the ether cells in Fig. 189 

 to be displaced in a manner to represent the tapering electric 

 field of Fig. 191. Then the two positive cells to the left of the 

 middle cell in Fig. 190 are displaced downwards more than the 

 two positive cells to the right of the middle cell in Fig. 190, so 

 that the two torques a and b exceed the two torques c and d, 

 that is, the middle cell is acted upon by an unbalanced torque, 

 and, therefore, the middle cell must be gaining angular velocity 

 about an axis perpendicular to the plane of the paper. What is 

 here said of this particular cell is true of all the ether cells in 

 Fig. 189 in the case of a tapering electric field; therefore, a 

 tapering electric field produces a continuously increasing magnetic 

 field at right angles to itself.* 



131. Electric action of a tapering magnetic field. Hereafter 

 a chain of geared cells which is free from distortion will be 

 thought of as standing in a straight row as shown in Fig. 1920, 



Fig. 192o. 



and the opposite displacements of positive and negative cells 

 which constitute an electric field will be thought of as changing 

 such a straight row to a zigzag row, as shown in Fig. 1926. Con- 

 sider a number of geared cells which tend by elastic action of 

 any kind to stand in a straight row like Fig. 1920. Such a row 

 is converted into a zigzag row as shown in Fig. 1926 if the cell a 

 is turned while the cell b is kept stationary. 



* This is a modified form of statement of the law of induced electromotive force. 

 It is a verbal statement of the differential equation 



dM _ dZ 

 dt " dx 



in which M is the y-component of magnetic field, and Z is the z-component of 

 electric field. One form of this differential equation is derived and discussed in Art. 

 116. 



