258 



ADVANCED ELECTRICITY AND MAGNETISM. 



drop along the ribbons, and the energy stream, instead of flow- 

 ing straight upwards, turns to some extent into the ribbons 

 where it appears as the RI 2 loss. This is shown roughly in 

 Fig. 200 which represents the flow of energy from a generator 

 along a transmission line AB to a distant lamp L. It is im- 

 practicable in this case to represent the exact trend of the lines 

 of force of the electric and magnetic fields in the neighborhood 

 of the generator and in the neighborhood of the lamp.* 



Fig. 200. 



(c) The flow of energy in an electromagnetic wave. The energy 

 in an electromagnetic wave flows continuously from the back 

 part of the wave to the forward part of the wave, in Fig. 194, 

 and this energy flow is due to the coexistence of electric and 

 magnetic fields at right angles to each other in the wave. 



135. The electric oscillator. A clear understanding of the de- 

 tails of action of the electric oscillator depends upon an insight into 

 what takes place when a condenser is charged and discharged. 

 Before discussing the electric oscillator, therefore, it is necessary 

 to consider the charge on a condenser and its mode of disappear- 

 ance when the condenser plates are connected by a wire. Con- 

 sider a closed (endless) chain of gear wheels AB, Fig. 201, 



* Some examples of the theorem of energy flow are given in Poynting's original 

 paper in the Philosophical Transactions for 1884. Some interesting examples of 

 Poynting's theorem are given by W. S. Franklin, in Physical Review, Vol. XIII, 

 pages 165-181, 1901. The details of field distribution and energy flow in the 

 neighborhood of two long parallel cylindrical conductors (line wires) are given by 

 G. Mie, Zeitschrifl fur Physikalische Chemie, vol. XXXIV, page 522. 



