CONTEMPORARY ADVANCES IN PHYSICS 577 



the electrodes in a characteristic way, and manifests itself by a striking 

 subdivision of the gas into zones of light and zones of darkness, 

 differing much from one another in their electrical state as well as in 

 their aspect. This distribution is not established instantly; would 

 one not do better to examine it at leisure and understand it in full for 

 each separate value of voltage, before beginning to study discharges 

 in which it is continually changing over from the form appropriate 

 to one voltage into the form appropriate to another? or discharges 

 in which the voltage is varying so rapidly, that the gas is always in a 

 state of transition, never even approaching the equilibrium appropriate 

 to any steady value of voltage whatever? 



Well, this is not necessarily to be supposed! it may be that when 

 the voltage across the gas varies with extreme rapidity, the gas itself 

 enters into a sort of equilibrium-state, perhaps even a more intelligible 

 state than that which it attains when it is allowed an ample time to 

 adjust itself to a constant value of voltage. The simplicity of certain 

 empirical laws of the high-frequency glow suggests this, as also does 

 the aspect of the glow. There is also the following argument, rather 

 paradoxical in sound, perhaps, but forcible. A self-sustaining direct- 

 current discharge, a glow or an arc, involves a steady outflow of 

 electrons from its cathode. This outflow must be maintained by 

 agents coming out of the gas itself — photons and positive ions and 

 excited atoms, which are generated in the gas by the discharge and 

 fall upon the cathode. Of the relative prominence of these agents, 

 little is known — it forms one of the major problems of the steady dis- 

 charge in gases; but at least it is certain, that they owe their existence 

 and their effectiveness largely to the distribution of space-charge in the 

 gas. The distribution of the space-charge therefore is controlled by 

 the requirement (or seems controlled — one never knows what is cause 

 and what is effect) that the electron-outflow from the cathode must 

 be kept at a level suitably high. Now in the high-frequency discharge, 

 the demand for electrons from the cathode is attenuated or abolished ; 

 witness the fact that such discharges may be maintained when the 

 electrodes are separated by insulators from the gas! The peculiar 

 disposition of the space-charge is therefore not demanded, at least not 

 to so great an extent; conditions are intrinsically simpler. 



This feature of high-frequency discharges — their competence to do 

 without electrons from the cathode — requires attention. It is derived 

 from a fundamental principle, which one is all too likely to forget 

 if one has long been occupied with steady currents: the principle of 

 Maxwell, that an electric field when varying in time is equivalent to 

 a current. Let us apply this principle to a current in a circuit (Fig. 1) 



