5 i2 THE POPULAR SCIENCE MONTHLY. 



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 Ruhm- 

 korff 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 in- 

 creased 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 maxi- 

 mum spark distance, and then, by constantly changing the capaci- 

 ty 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 prin- 

 ciple 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 centi- 

 metres directly over the discharger, and then continues in a 

 straight horizontal line some sixty metres. The end is left free, 



