222 hertz's experiments. 



■w^ith the generator, only that the sx^ark gap in the generator shonlcl 

 be represented by a connecting wire. They may be longer and thin- 

 ner. If longer, they should be thinner, or they will not have the 

 same period of vibration. On the whole, the best results have been 

 got with conductors somewhat longer and thinner than the generator. 

 It is not generally convenient that the spark between the two conduc- 

 tors that form the detector should take idace directly from one to the 

 other. It is not easy to make arrangements by which the distance apart 

 of these conductors can be regulated with sufficient accuracy. The most 

 convenient way is to connect the lower end of the upper conductor and 

 the upi^er end of the lower one each with a short thin wire leading, one 

 to a fixed small knob and the other to a very fine screw impinging on 

 the knob. The screw may then be used to adjust the spark gap be- 

 tween it and the small knob Avith great accuracy. This spark gap 

 must be very small indeed, if delicate work be desired. A thousandth 

 of a centimeter would be a fair-sized spark gap. The minute sparks 

 that are formed in these gai)S when doing delicate work are too faint 

 to be seen, except in a darkened room. Having placed the detector in 

 positiori between the generator and the screen, the difficult part of the 

 observation begins. It is heart-rending work at first. A bright spark 

 now and then arouses hope, and long periods of darkness crush it again. 

 The knobs of the generator require re-polishing; the spark gap of the 

 detector gets closed up: dust destroys all working, and not without 

 much patience can the art be attained of making sure of getting sjtarks 

 whenever the conditions are favorable, though it is easy enough not to 

 get sparks when the conditions are unfavorable. 



Before making any measurements all this practice must be gone 

 through. It is hard enough with the success of others before us to en- 

 courage us, with their advice to lead us, with a clear knowledge of 

 what is to be exi>ected to guide us. How much credit then is due to 

 Hertz, v;ho groped his way to these wonderful experiments from step 

 to step, without the success of others to encourage him, without the 

 advice of others to lead him, without any certainty as to what was to 

 be expected to guide him. Patiently, carefully, through many by- 

 paths, with constant watchfulness, and checking every advance by re- 

 peated and varied experiments. Hertz worked up to the grand sim- 

 plicity of the fundamental experiment in electricity that is engaging 

 our attention. 



Having gained conmiand over the apparatus we may look about for 

 places where sparks occur easily and for others where they can not be 

 produced. Two or three places may be found where no sparks can be 

 observed. These places will be found to be nearly equi-distant. They 

 are the nodes we are in search of. The distance between any pair is 

 half the distance an electric wave is propagated during the period of 

 an oscillation. Their presence proves that the electric force is not prop- 

 agated instantaneously, but takes time to get from place to place. If 



