ANNIVERSARY ADDRESS. 17 
tions of widely different periods if only it be of such a type that 
when speaking itself it is heavily damped. The sounding board 
of a piano or the body of a violin, or indeed the air in the ear 
passages of any animal, is such a body. If on the other hand the 
resonator be only slightly damped, it will require much greater 
accuracy of tuning to set up resonance, but when once this is 
attained the resonance will be much stronger. In acoustics a 
tuning fork or closed organ pipe is a resonator of the class to 
which I allude. In the electrical case, so far as I understand it, 
the matter is made more complex by the fact that both electric 
and magnetic energy are radiated, and one kind of radiation may, 
and generally does, proceed faster than another, so that a vibrator 
may be heavily damped as far as electrical radiation is concerned, 
and only slightly so far as magnetic energy is concerned, and 
vice versa. Ultimately the energy per unit volume of both kinds 
will be equal in the wave field, but meanwhile the effects may 
become too attenuated for observation. We owe a good deal to 
Bjerknes* in this connection. A clear but brief account is also 
given of the leading results in the Archives des Sciences Physiques 
et Naturelles de Gentve—a most useful and pleasant publica- 
tion. By the application of the principles just referred to, the 
study of electro-magnetic waves as produced by resonators of 
varying form is greatly assisted, and most if not all of the out- 
Standing difficulties have now been removed. This brings me 
naturally to a consideration of the brilliant work of Nikola Tesla. 
in this field—work which exhibits the highest genius at every 
Step. The advance made by Tesla is briefly this. Hertz worked 
with comparatively short waves which damped down after a very 
few vibrations, occupying a time perhaps in all of the order of the 
ten millionth of a second. With the apparatus used by Hertz 
the phenomena only occurred perhaps forty times a second, so 
that the time occupied by the phenomenon itself was only a very 
small fraction of the whole time involved, just as if one were to 
ring a bell for a minute every year and endeavour to deduce 
ee ai Un i eae 
* Wied. Ann. 44, p. 74. 
B—May 1, 1895, 
