PRINCIPLES OF RADIOLOGICAL PHYSICS 15 



radiation originates from a change of intensity of an electric current in the 

 course of time, like the more familiar phenomenon of electromagnetic 

 induction. In fact, electromagnetic radiation may be regarded as a 

 persistent and far-traveling offshoot of ordinary induction. A brief 

 review of the phenomenon of induction yields some indication of the 

 mechanism of electromagnetic radiation and of the main factors that 

 influence its production. 



The onset of a current in an electric circuit is never abrupt but is marked by a 

 lag due to the phenomenon of "self-induction." During this lag the current 

 generator performs work to establish the current against the opposition of self- 

 induction. As the current is established, the surrounding space becomes the 

 seat of magnetic forces. 



The whole phenomenon is reversed when the current generator is switched off ; 

 self-induction keeps the current running for awhile, thus returning to the current 

 the extra energy that had been spent in setting it up. The current intensity as 

 well as the magnetic forces dies off gradually. 



Both the starting and the disappearance of magnetic forces in the space sur- 

 rounding the wire are associated with further electric actions. Thus an electric 

 current is driven temporarily in a closed loop of wire lying next to the wire in 

 which a current starts or dies off. This effect of electromagnetic induction under- 

 lies the common a-c transformer action, in which an oscillating current is pumped 

 in a coil of wire (the "primary" winding) and another current is obtained in 

 another coil (the "secondary" winding). 



Little energy is dissipated in the course of induction phenomena if the rates of 

 change of current intensity are kept rather low, as in ordinary a-c phenomena. 

 Roughly speaking, the dissipation remains low because the electromagnetic 

 equilibrium in the space surrounding a current is never greatly disturbed by 

 rather slow variations of the current. This does not hold true in the case of 

 slmrp current variations. A mechanical analogy may help to illustrate this 

 l)oint. A slow compression of gas in a cylinder proceeds reversibly without dis- 

 sipation of energy, but a substantial amount of energy is dissipated into sound 

 waves if the gas is acted upon by a rapidly oscillating piston. 



Similarly, rapid variations of electric currents lead to a substantial energy 

 dissipation. The situation may be visualized thus: the more rapid the current 

 variations, the less readily can the magnetic and electric forces in the surrounding 

 space readjust to follow the current variations. This unbalance leads to a larger 

 energy transfer away from the current. It looks as though a very rapid increase 

 of current requires an especially large expenditure of energy, but a very rapid 

 decrease yields an especially low return of energy. The reactions caused by the 

 electromagnetic unbalance in the surrounding space overshoot the equilibrium, 

 just as it happens in the phenomena of elastic unbalance, and thus the disturbance 

 propagates farther and farther away. 



In conclusion, it may be said that electromagnetic radiation arises from 

 the induction effects associated with any variable current. The energy 

 radiated by a current is an increasing function of the rate of variation of 



