thirty centimetres diameter were substituted, and these were movable along the rods. As these constitute the electrical ends of the discharger, the same may be altered in length by the total diameter of each by simply letting the rods project into the cavity of the spheres. The time of oscillation of the waves in the Ruhmkorff can thus be altered. The brass balls of the rectangle were provided with a micrometer adjustment, so that the length of spark which passed might be measured. The connecting wire was in these experiments dispensed with, and the rectangle was mounted on insulators in front of the Ruhmkorff discharger.
With this arrangement Hertz carried out a complete set of observations, in each of which the effect of a regular series of changes in one of the variables was investigated—e. g., the time of oscillation of the primary discharger would be regularly increased by changing the capacity or self-induction, and for each change the length of spark in the rectangle would be measured. One series in detail will suffice for our purpose.
Suppose, at the beginning of the experiment, that the time of oscillation of the rectangle is smaller than that of the Ruhmkorff discharger, and the spark is one millimetre long. If now we hang two hooks of wire on each ball of the rectangle, the capacity is increased, and we get a spark of three millimetres. Add two more equal hooks, and the spark is five millimetres. Add two more, and it falls off to three millimetres again. If this process be continued, the spark will alternately reach a maximum and minimum, and the natural inference is that the time of oscillation of the rectangle is nearest that of the Ruhmkorff discharger when the spark in the former is at a maximum.
Perhaps it is most striking to place the micrometer at the maximum spark distance, and then, by constantly changing the capacity of either conductor, cause the spark to disappear and reappear. Should small spheres be used, instead of wires, for changing the capacity, we would then have a direct means of determining the wave-length.
These sets of experiments led Hertz to conclude that the principle of resonance is as true for electrical waves as for sound waves, and he employs it for his quantitative work.
The arrangement of apparatus is as follows: To the outer ends of the Ruhmkorff discharger are attached two plates, whose planes are vertical and embrace the line of direction of the discharger. Back of one of these is mounted on an insulated stand a similar plate of the same size. A wire leads from the inner central edge of this to a point on a level but just back of the air-space of the discharger. It then turns in a curve to a point about thirty centimetres directly over the discharger, and then continues in a straight horizontal line some sixty metres. The end is left free,