PRESIDENTS ADDRESS-—SECTION A. AD 
step is to find some means of detecting the electromagnetic waves. 
which, according to our theory, are “propagated outwards from 
the vibrator. ‘This Hertz accomplished by taking advantage of 
a property borrowed from acoustics. It is well known that if 
any vibrating system is subject to accelerating forces of the same 
period as its own period of vibration, when vibrating freely, the 
effect will be cumulative, and the system will be caused to 
vibrate strongly, though any individual impulse might be quite 
incapable of producing an observable effect. The same principle 
may be extended to the action of electric oscillations on con- 
ductors. We must have a “resonator” or conductor where 
natural period of electrical oscillation calculated from Sir William 
Thomson’s formule coincides with that of the vibrator. In 
practice, the length of wire most appropriate to the resonator is 
found by experiment. The wire is bent into a circle and the 
ends brought close together by a fine screw attachment. If 
electric forces act on such a resonator in such a way as_ to 
produce a cumulative effect the electrical disturbance will 
become sufficient to cause sparks to pass between the ends of 
the wire. We have therefore both a means of setting up 
disturbances of the required character and of detecting them 
at a considerable distance away. <A vibrator of the dimensions 
given above completes an oscillation in one-thirty-millionth of 
« second, and if the disturbance is propagated with the velocity 
of light will consequently yield waves of about ten meters wave 
length. The resonator for such waves as these was found to 
require two hundred and ten centimetres of No. 17 wire when 
bent into a circle. Without going into many very interesting 
questions as to the best relative positions of the planes of vibrator 
and resonator it will be sufficient to state that in one position 
of the resonator the most effective component is the electric, 
and in the perpendicular position the most effective component 
is the magnetic intensity. Perhaps the most important experi- 
ment one may make with this apparatus is the demonstration of 
nodes and loops between the vibrator and a large sheet-zinc 
reflector. The length of the waves roughly confirms the theory 
that the velocity of propagation is the velocity of light, while 
the existence of loops and nodes demonstrates the truth of the 
more important preliminary assumption as to the existence of a 
medium. The apparatus itself may be modified and for some 
purposes improved by using two cylinders tipped with balls for 
the vibrator and placing them in the focus of a large parabolic 
cylindrical mirror so as to render the electric rays parallel. The 
receiver in this arrangement consists of a lengthy wire placed on 
the focal line of another mirror and interrupted by a spark gap 
in the usual manner. With this apparatus Hertz has imitated 
most optical effects. He has shown that the ordinary laws of 
reflexion of light are obeyed by these electric or “etheric ” waves, 
