ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 245 



138. The energy stream in the electromagnetic field. A 



region in which electric field and magnetic field co-exist may be 

 called an electromagnetic field for the sake of brevity. It has 

 been shown by J. H. Poynting * from theoretical considerations 

 that energy streams through an electromagnetic field in a direction 

 which is at right angles both to the electric field and to the mag- 

 netic field at each point, and that the amount of energy per second 

 which streams across one square centimeter of a.rea is propor- 

 tional to the product of the electric and magnetic field intensities. 

 In case the electric and magnetic fields are not at right angles to 

 each other, the energy stream is proportional to the product of 

 the intensities of the two fields and the sine of the included angle. 

 Conception of the energy stream. Consider a row of gear 

 wheels as shown in Fig. 176. Imagine the wheel W to bq 



frank 



bxbke 



Fig. 176. 



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

 by a brake. The result is that energy is continuously trans- 

 mitted along the chain of gear wheels from W to W, any 

 given gear of the chain is acted upon by equal and opposite 

 torques by the gear wheels on each side of it, the transmission 

 of energy by the chain depends upon this torque action combined 

 with the motion of the wheels, and the rate at \vhtciT~energy is 

 transmitted along the chain is proportional to the product of the 

 speed of the wheels and the torque action between adjacent wheels. 

 Imagine the ether cells in Fig. 174 to be rotating, positive cells 

 in one direction, negative cells in the other, about axes perpen- 

 dicular to the plane of the paper. This rotatory motion consti- 

 tutes a magnetic field perpendicular to the plane of the paper and 

 perpendicular to the electric field which is towards the bottom of 



*See Philosophical Transactions, Vol. 175, Part II, page 343, 1884. 



