THE IDENTITY OF LIGHT AND ELECTRICITY. 185 



millionth of a second is no longer a short interval. But they 

 would be of little use if we were not able to compare them at 

 that distance of about ten metres which we have proposed to our- 

 selves. The means employed for this purpose are very simple. 

 We fix a conductor for instance, a straight metallic wire, having 

 a slight interruption at one point at the place where we desire to 

 perceive the signal. When the electrical field is rapidly varied, a 

 spark appears in the conductor. 



The means of observation could be pointed out only by experi- 

 ment. Theoretically it was hard to imagine it. The sparks are, 

 in fact, microscopic, being hardly a hundredth of a millimetre 

 long, and they continue less than a millionth of a second. It is 

 extremely hard to conceive them as visible. Yet they can be seen, 

 in a dark room and by an eye at rest. On so light a thread is 

 hung the success of our undertaking. We had in the beginning 

 a number of questions to answer. Under what conditions are the 

 vibrations strongest ? We must try to secure those conditions. 

 What form should the conductor have ? The phenomena will 

 vary as we use straight or bent wires, or conductors of other 

 forms. The form being determined upon, of what size should our 

 conductor be ? This is not a matter of indifference, for we shall 

 see that we can not study all the vibrations with the same con- 

 ductor. There are relations between the two elements like the 

 phenomenon of resonance in acoustics. Lastly, in how many dif- 

 ferent positions can we arrange this conductor ? We shall see 

 the sparks at times increase in intensity, or become weaker, or 

 disappear, I can not enter into these details ; they are simply 

 accessory to the theory as a whole. They are of importance only 

 to the operator, and are simply properties of his instrument. 



What the experimenter will educe from his process will de- 

 pend on his knowledge of his means of action. The study of the 

 instrument and the answers to the questions I have just men- 

 tioned therefore formed the most considerable part of my labor. 

 This task having been disposed of, the solution of the problem 

 was before me. 



A physicist, given a number of diapasons and resonators, will 

 find no difficulty in demonstrating that sound is not propagated 

 instantaneously, even in the restricted space of a room. Having 

 set the diapason in vibration, he goes with his resonator to differ- 

 ent parts of the room and observes the intensity of the sound. 

 He perceives that it becomes weak in some places, and infers 

 from this that each vibration is annulled by another of later ori- 

 gin, which has reached the spot by a shorter route. If less time 

 is taken in traversing the shorter road, propagation is not in- 

 stantaneous, and the question is answered. But our physicist 



will then show us that the points of silence succeed one another 

 vol. xxxvin. 13 



