CONTEMPORARY ADVANCES IN PHYSICS 



605 



// standing as usual for the magnetic field strength. Since this 

 frequency remains the same however much the speed of the electron 

 may change, and therefore is the same for all the electrons in the 

 region in question and for all values of electric field strength, we may 

 expect it to be important when an ionized gas (or a volume containing 

 free electrons but no atoms) is exposed to a high-frequency field; 

 in curves of dielectric constant and conductance vs. frequency, we 

 may look for peculiarities when v = vh. The precise theory, I must 

 add, is not simple when collisions of electrons with atoms must be 

 taken into account (the fundamental equations were given long ago 

 by Lorentz); but under certain restrictions — according to Appleton 

 and Childs, the number of electrons per cc. and the number of collisions 

 per cycle of the high frequency must be kept under certain limits — 

 it leads to the inference that there should be a maximum of conductance 

 at V = VH- 



This maximum is manifested by the sharp and striking minimum 

 seen near the middle of the curve in Fig. 11. For obtaining these 



20 25 



H IN GAUSS 



Fig. 11 — Evidence of a natural frequency produced in an ionized gas by a 

 constant magnetic field. (Appleton & Chapman.) 



data, Appleton and Childs had an arrangement similar in the main to 

 that of Fig. 5, excepting that the detecting galvanometer was coupled 

 (through an amplifier) between the far ends of the wires of the bridge, 

 and there was a magnetic field of adjustable strength parallel to the 

 axis of the tube. Though I have spoken thus far of what should be 

 observed when H is held constant and v is varied through the value 

 given by equation (26), it is more convenient in practice to hold the 

 frequency constant and vary H through the corresponding value. 

 The curve of Fig. 11 is accordingly a curve of galvanometer-reading 



