hertz's researches on electrical waves. 153 



directly to the secondary terminals of the induction coil. Two hollow 

 zinc spheres, 30 centimeters in diameter, were made to slide on the wire, 

 one on each side of the discharger, and since, electrically speaking, 

 these formed the terminals of the conductor, its length could be varied 

 by altering their position. The micrometer circuit was chosen of such 

 dimensions as to have, if the author's hypothesis were correct, a slightly 

 shorter vibration period than that of C C. It was formed of a square, 

 with sides 75 centimeters in length, of copper wire 2 millimelers in di- 

 ameter, and it was placed with its nearest side parallel to C B C, and 

 at a distance of 30 centimeters from it. The sparking distance at the 

 micrometer was then found to be 0.9 millimeter. When the terminals 

 of the micrometer circuit were placed in contact with two metal spheres, 

 8 centimeters in diameter, supported on insulating stands, the sparking 

 distance could be increased up to 2.5 millimeters. Wlien these were re- 

 placed by much larger spheres the sparking distance was diminished to 

 a small fraction of a millimeter. Similar results were obtained on con- 

 necting the micrometer terminals with the plates of a Kohlrausch con- 

 denser. When the plates were far apart the increase of capacity 

 increased the sparking distance, but when the plates were brought close 

 together the sparking distances again fell to a very small value. 



The simplest method of adjusting the capacity of the micrometer cir- 

 cuit is to suspend to its ends two parallel wires, the distance and lengths 

 of which are capable of variation. By this means the author succeeded 

 in increasing the sparking distance up to 3 millimeters, after which it 

 tliminished when the wires were either lengthened or shortened. The 

 decrease of the sparking distance on increasing the capacity was natu- 

 rall.v to be expected ; but it would be difficult to understand, except on 

 the principle of resonance, why a decrease of the capacity should have 

 the same effect. 



The experiments were then varied by diminishing the cai)acity of the 

 circuit C B C so as to shorten its period of oscillation, and the results 

 contirmed those previously obtained, anda series of experiments in wiiieh 

 the lengths and capacities of the circuits were varied in dilfeient ways 

 showed conclusively that the maximum effect does not <lepend on the 

 conditions of either one of the two circuits, but on the existence of the 

 proper relation between them. 



When the two circuits were brought very close together, and the dis- 

 charger knobs separated by an interval of 7 millimeters, sparks were 

 obtained at the micrometer, which were also 7 millimeters in length, 

 when the two circuits had been carefully adjusted to have the same pe- 

 riod. The induced electro-moHre forces must in this case have attained 

 nearly as high a value as the inducing ones. 



To show the effect of varying the co-efficient of self-induction, a series 

 of rectangles abed (Fig. 4), were taken, having a constant breadth 

 a h, but a length a c continually increasing from 10 centimeters up to 

 250 centimeters ; it was found that the maximum effect was obtained 



