ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 253 



to p f around the end AB as shown by the curved line in Fig. 

 196. The current in AB is assumed to be maintained indefi- 

 nitely, and therefore the opposite rotation of p and p f is 

 assumed to continue indefinitely, but since this continued opposite 

 rotation of p and p' cannot 

 be accommodated by ever- 

 increasing distortion of the 

 elastic gear teeth of the ether 

 cells along the chain which 

 passes around the end of A B, 

 a slip must take place be- 

 tween adjacent cells at some 

 point along this chain. There- 

 fore the line of flow of the current A B (line of slip of geared 

 cells) must form a closed circuit which cuts across every possi- 

 ble chain of geared cells extending from p to p f . When a cur- 

 rent flows along a path which does not form a closed circuit, 

 then an increasing ether distortion (electric field) is produced 

 around the end portions of the path as explained in Art. 135. 



134. The flow of energy in the electromagnetic field. Poyn- 

 ting's theorem. It has been shown by J. H. Poynting* from 

 theoretical considerations that energy streams through an electro- 

 magnetic field in a direction at right angles both to the electric 

 field and to the magnetic field at each point, and that the amount 

 of energy per second which streams across one square centimeter 

 of area is proportional to the product of the electric and magnetic 

 field intensities. If 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. 197. Imagine the wheel W to be turned 



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

 original paper by Poynting is well worth reading. 



