254 ADVANCED ELECTRICITY AND MAGNETISM. 



steadily by a crank, and the wheel W to be hindered by a brake. 

 Under these conditions energy would be continuously transmitted 

 along the chain of gear wheels from W to W. Each wheel of 

 the chain would be acted upon by equal and opposite torques by 

 the wheels on either side of it, the transmission of energy along 

 the chain would depend upon this torque action and the rota- 

 tory motion of the wheels, and the rate at which energy would 



Fig. 197. 



be transmitted along the chain would be proportional to the 

 product of the speed of the wjheels and the torque action between 

 adjacent wheels. 



Imagine the ether cells in Fig. 189 to be rotating, positive cells 

 in one direction and negative cells in the other direction, about 

 axes perpendicular to the plane of the paper. This rotatory 

 motion constitutes a magnetic field perpendicular to the plane 

 of the paper and perpendicular to the electric field which is 

 towards the bottom of the page in Fig. 189. On account of the 

 torque action between the cells (as explained in connection with 

 Fig. 190) combined with the rotation of the cells, energy is trans- 

 ferred to the right (or left) by each horizontal chain of geared 

 cells in Fig. 189 at a rate which is proportional to the product of 

 the intensity of the magnetic field and the intensity of the 

 electric field; and the energy per second flowing across an area 

 (at right angles to both electric and magnetic fields) is propor- 

 tional to the product of the respective field intensities and pro- 

 portional to the area, inasmuch as the area determines the num- 

 ber of rows of cells which participate in the transfer of the energy. 



Examples of Poynting's theorem, (a) The flow of energy in 

 the neighborhood of a wire carrying an electric current when no 

 electric charge resides on the surface of the wire. In general, the 



