of Secondary Electromagnetic Waves. 271 



to the previous swing. This will be better understood by 

 recollecting that at the conductor, when the electric force is 

 at its greatest development, the current is zero, and conse- 

 quently so, too, the magnetic force, while in radiation the 

 electric force and the magnetic force simultaneously reach 

 their maximum value. This is beautifully shown in Hertz's 

 diagrams. In fig. 1, at the centre the electric force is zero, 

 while the magnetic force is about at its greatest development. 

 The latter travels outwards with such a velocity at each point 

 (the same as that of the circular line of force in the diagrams) 

 that there is an acceleration of \ period ; while the former 

 vanishes at the centre but reappears at A/4'4, as is seen in 

 fig. 4, so as to be now one quarter wave-length behind. In 

 this w r ay the necessary readjustment of the relative phases 

 of the electric and magnetic components is effected. 



If the position on the wave of maximum or minimum value 

 of z (that is to say, the " crest ") be taken instead of the zero 

 value, the condition at any point is that 



tan (mr — nt) = > 



and the same expression for the velocity as before is obtained, 

 so that the position at distance X/4'4 may be considered the 

 point from which the electric disturbance originates, and not 

 the centre. As the energy in the neighbourhood of the con- 

 ductor turns from its magnetic to its electric form*, it is at this 

 point that the maximum value is first reached, and is in 

 advance of the " normal epoch " t/4, or that of the " crest " 

 at the centre by about ^ t. The maximum value spreads both 

 outwards and inwards from A/4'4, reaching the centre at t/4, 

 the epoch of fig. 3. 



As in the case of the zero value of z, so here also, if the 

 period elapsing between the arrival at any point and the time 

 of greatest electric development at the centre, which occurs 

 at the epoch of fig. 3, be calculated, an acceleration of t/2 on 

 the normal time is found. This virtual acceleration of t/2 is, 

 however, due to the combined effect of the " crest" or maxi- 

 mum value of z, (1) starting from the point X/4'4 instead of 

 from the centre, (2) at a time about J t previous to the 

 normal epoch t/4 or that at the centre, (3) as well as to the 



* In the curves obtained by plotting % for a series of epochs the forma- 

 tion of the wave at X/4*4 is clearly observed. The wave after springing 

 here into existence above the zero-line, lengthens out in both directions, 

 and then beginning to sag in the centre, finally splits into two at this 

 point, as the force reaches zero, again to pass over to the other side of 

 the zero-line. 



