214 Effect of Ionization of Air on Electrical Oscillations. 



it is subjected to an alternating force, which causes it to 

 oscillate about a mean position. If it is entirely free from 

 frictional restraint, the acceleration in any direction will be 

 a maximum when the force and therefore the field is a 

 maximum in that direction. The molecule must have its 

 maximum displacement in the opposite direction at this 

 moment, and the electric current due to its motion will 

 therefore lao- 90° behind the electric held. This current 

 is therefore in exact opposition to the pure displacement 

 current, and has the effect of reducing the apparent di- 

 electric constant of the medium. The Telocity of propagation 

 of electromagnetic waves should therefore be increased. 



If the molecular motion is not free from frictional restraint, 

 the current will not be in exact quadrature but will have a 

 power-component, the relative magnitudes of the two com- 

 ponents being dependent, among other things, on the 

 freedom of molecular motion, that is, on the vacuum. 



This phenomenon must not be confused with the com- 

 paratively well-known peculiarities of leaky condensers, 

 i. <?., condensers shunted with a non-inductive resistance. 

 Here the leakage current is in phase with the electric 

 force, and therefore in quadrature with the displacement 



current. If R, L, 0, and -a represent the resistance, 



inductance, capacity, and shunt resistance of an oscillatory 

 circuit, we have 



- f -2irV LC \2L 20/ * 



As the leakance S is increased from zero it counteracts 

 the effect of R in decreasing the natural frequency, until, 



when S = -y-, the frequency reaches a maximum. Further 



increase in S causes the frequency to decrease. This has no 

 connexion, however, with the type of: conductivity con- 

 sidered above. 



Ordinary leakage cannot increase the velocity of propagation 

 of electromagnetic waves. 



This is readily seen from the formula for the velocity of 

 propagation along a telephone line, which is 



CO 



where co = 2irf 



and 2q~ = ^(R J + »^«XS» +*»<?) - RS + arTL, 



