WIRELESS TRANSMISSION OP ENERGY THOMSON. 249 



mary circuit) to the other (the secondar}^ circuit) may be accom- 

 plished. But in the wire itself, which leads from an alternating- 

 current source, since there is an action called a current which changes, 

 pulsates, or alternates, we have also around the wire core waves in 

 the ether which, in fact, spread to very great distances; some small 

 portion of the energy of each impulse not returning to the system, 

 but passing outward into space as radiated energy. 



This radiation may be a very small amount per cycle, especially 

 where the outgoing and return wires are near together and parallel, 

 and with low frequencies, such as 60 cycles, on account of the low 

 number of waves per second and the low speed or rate of change in 

 the fields surrounding the wire, the amount of energy carried oif by 

 free radiation into space is indeed negligible. But if we raise the 

 frequency we raise the amount of energy which can be radiated pro- 

 portionately to the number of waves per second, and we also make 

 the rate of change higher and the wave slopes steeper, so that 

 as the frequency rises the radiation factor becomes more and more 

 important in dissipating the energ}^ of the system. It will be noticed, 

 however, that such energy is not directed energy. It is diffused 

 through space around the electric system at work and passes off to 

 illimitable distances. Since these impulses in the wire, the electrical 

 waves sent along the wire (with the wire as a guiding core), can at 

 the maximum move with the speed of light — 186,000 miles per sec- 

 ond — it follows that if the line is sufficiently long or the transmis- 

 sion sufficiently extended or the path of radiation sufficiently distant 

 the wave stresses or fields or currents can exist at different parts of 

 the system in phases either much displaced or entirely opposite. 

 This may be rendered clear by stating that while one portion of a 

 very long line might be positive to earth another portion half a 

 w\ave length distant from the first along the same line would be 

 negative to earth (fig. 12). In other Avords, there may exist upon 

 the system at the same instant a succession of waves in opposite 

 phase. Just as in vibrating strings in musical instruments or vibrat- 

 ing columns of air in organ pipes there are stationary waves, nodes, 

 and internodes, so in electrical systems in vibration there can be 

 nodes and internodes if the conditions are selected for obtaining that 

 effect. Here the dotted vertical line indicates the nodes of the 

 waves. We may thus have so-called stationary electric waves 

 (fig. 12). 



We find that on raising the frequency of an alternating-current 

 system from, say, 60 cycles, the ordinary frequency, to 600 cycles, an 

 effect which at first was hardly detectable now becomes important. 

 It is the so-called " skin effect " whereby the current in a wire cir- 

 cuit tends to concentrate itself on the outer skin of the conducting 

 wire, neglecting the inner copper, so that the inner core of the wire 



