514 Mr. E. Edser on the Reflexion of Electromagnetic 



Obviously it is best to express the pressure in terms of the 

 energy per unit volume of the incident wave-train, since this 

 quantity remains constant. 



6. If we wish to express the pressure exerted on the 

 mirror in terms of the force exerted on the superficial 

 screening current induced in the mirror, we must take 

 account of the fact that when a stationary magnet pole is 

 placed in the path of a wave-train, the force exerted on it 

 is not in general equal to that which acts when the pole is 

 moving. Rowland's famous experiment has shown that an 

 electric field, moving relatively to a magnet pole, exerts a 

 force on the latter ; hence it follows that when a magnet pole 

 moves relatively to an electric field, a similar force will be 

 exerted on the pole. The following rule may be employed to 

 determine the direction of this force. Let a watch, indicating- 

 three o'clock, be placed with the minute-hand pointing in the 

 direction of the electric field. Then if a unit N pole is moved 

 in the direction in which the hour-hand points, the force 

 exerted on the pole will act from front to back of the face. 

 If the electric field-strength is equal to E, and the velocity 

 of the pole is equal to u, the magnitude of the force exerted 

 on the pole is equal to Ei^/c 2 , where c denotes the velocity of 

 light*. 



Thus it follows that a unit N pole, placed just in front of, 

 and moving with the mirror, is subjected to a force directed 

 from South to North, of magnitude {H-f E(v/c 2 )}, where H 

 denotes the instantaneous value of the force that would be 

 exerted on a stationary unit N pole at the same place, and 

 E denotes the instantaneous value of the force that would 

 be exerted on a stationary unit positive electric charge. 

 Expessing this result in terms of the electric fields due to the 

 incident and reflected waves, we find that the force on a 

 unit N pole moving with the mirror 



= ^(E 1 -E 2 )+3(E 1 + E 2 )=J{(c + t ) )E 1 -(c-«)E 2 }. (8) 



C C O 



Now describe a rectangular circuit in a horizontal plane, 

 one side of the circuit being parallel to and just outside the 



* The auxiliary electric and magnetic forces due to motion were con- 

 sidered by Heaviside in 1885 (Electrical Papers, vol. i. p. 446). In 

 Heaviside's vectorial notation, the auxiliary electric force e = VuH, 



while the auxiliary magnetic force h= -^VEu, where H and E denote 



the vector forces that would act on a stationary unit N pole and a 

 stationary unit positive charge respectively, and u denotes the vector 

 velocity. 



