PRESIDENTS ADDRESS——-SECTION A. 433 
is attained. The period of oscillation is fixed by the electro- 
magnetic properties of the system, and is approximately—when 
the resistance is very small—T = 2 7 + a/ a seconds, when L- 
is selfinduction and C is capacity. This is the time that elapses. 
between consecutive similar electric states. The full expression 
% At OR? Awe: 
is T=2 7 = C ~_ which is homogeneous. Consequently 
oY 
the frequency of a Leyden jar discharge comes to be f= a/ er 
when the dissipation is small. 
Now Land C asa rule are very small quantities, so that the 
frequency is very high. At each oscillation, as has been said, we 
have tubes of induction moving once backward and once forward 
across the field. Now, there is nothing to prevent these tubes from 
radiating into space—if we are dealing with a medium we shall 
have an electro-magnetic disturbance which will continually pro- 
pagate itself outward if there is no dissipation, and its velocity 
will be comparable probably with the velocity of light. The wave 
length will be the distance moved by the tubes during one 
oscillation, and consequently will be given by dividing the velocity 
») (3 
by-the frequency, in fact A= ah Now the velocity is very high, 
so that though the frequency is very great the waves may still be 
extremely long. If we use Maxwell’s theory, then the velocity is. 
actually given by 1/ pK as has already been pointed out, and 
LC 
pk 
Clearly, then, a good way of testing Maxwell’s theory will be to 
get oscillations, and then measure the wave length by the dis- 
turbance propagated outward from them. The usual plan of esti- 
mating wave lengths is to produce a state of stationary vibration, 
and then measure the distance between nodes or planes of null 
effect. The distance from one node to another is always half a wave 
length, so that the measurement of the distance from node to node 
gives us a wave length on multiplication by two. The usual way 
of setting up a state of stationary vibration is to take advantage of 
the principles by interference. Every musical instrument is an 
illustration of this. The simplest way, of course, to set up a reflector 
and get interference between direct and reflected waves. It 
must always be borne in mind that no interference phenomena 
are possible at all unless waves take some time, however 
short it may be, to travel a finite distance, and consequently no 
instantaneous propagation theory could lead us to expect to 
observe this phenomenon. If, however, waves can be shown to 
interfere, we know that they must be propagated with a finite 
our expression of the wave length becomes A= 27 
