HERTZIAN WAVE WIRELESS TELEGRAPHY. 105 



mately the form of one line of electric strain round a linear oscillator, 

 with spark gap in the center, one half being charged positively and the 

 other negatively. Let us then suppose that the insulation of the spark 

 gap is destroyed, so that the opposite electric charges rush together and 

 oscillate to and fro. The strain lines at each oscillation are then 

 crossed or decussate, and the result, as shown in Fig. 2, d, is that a 

 portion of the energy of the field is thrown off in the form of self- 

 closed lines of strain (see Fig. 2, e). At each oscillation of the 

 charges the direction of the lines of strain springing from end to end 

 of the radiator is reversed. It is a general property of lines of strain, 

 whether electric or magnetic, that there is a tension along the line and 

 a pressure at right angles. In other words, these lines of electric 

 strain are like elastic threads, they tend to contract in the direction of 

 their length and press sideways on each other when in the same direc- 

 tion. Hence it is not difficult to see that as each batch of self-closed 

 lines of strain is thrown off, the direction of the strain round each 

 loop is alternately in one direction and in the other. Hence these 

 loops of electric strain press each other out, and each one that is formed 

 squeezes the already formed loops further and further from the radia- 

 tor. The loops, therefore, march away into space (see Fig. 2, /). If 

 we imagine ourselves standing at a little distance at a point on the 

 equatorial line and able to see these loops of strain as they pass, we 

 should recognize a procession of loops, consisting of alternately directed 

 strain lines marching past. This movement through the ether of self- 

 closed lines of electric strain constitutes what is called electric radia- 

 tion. 



Hence along a line drawn perpendicular to the radiator through 

 its center, there is a distribution of electric strain normal to that line, 

 which is periodic in space and in time. Moreover, in addition to these 

 lines of electric strain, there are at right angles to them another set of 

 self-closed lines of magnetic flux. Alternated between the instants 

 when the electric charges at the ends of the radiator are at their maxi- 

 mum, we have instants when the radiator rod is the seat of an electric 

 current, and hence the field round it is filled with circular lines of 

 magnetic flux coaxial with the radiator. As the current alter- 

 nates in direction each half period, these rings of magnetic 

 flux alternate in direction as regards the flux, and hence we 

 must complete our mental picture of the space round the radiator 

 rods when the charges are oscillating by supposing it filled with con- 

 centric rings of magnetic flux which are periodically reversed in direc- 

 tion, and have their maximum values at those instants and places where 

 the lines of electric strain have their zero values. Accordingly, along 

 the equatorial line we have two sets of strains in the ether, distributed 

 periodically in space and in time. First, the lines of electric strain 



