hertz's kxperiments. 219 



diictors simuitaueously, and tlic ii]»i)er ends (jf both will be similarly 

 electrified at any instant, wliilc the lower end of the n[)])er one will 

 always be oppositely electritied to the upper end of the low conductor, 

 and if these two points, or two short wires connected with them, be 

 close enough together, a spark Avill pass from one to the other whenever 

 the electric force sets up these electric oscillations in the conductor. 

 Thus this apparatus is a detector of the electric force. Whenever there 

 is a spark we may be sure that there is electric force, and whenever we 

 can not get a spark we may be sure that there is either no electric force 

 or at any rate too little to produce sparks. The apparatus will be more 

 sensitive for electric forces that oscillate at the same rate as the natural 

 vibration of the electric charge on the conductor, because the effect of 

 each impulse will then add to that of the last; resonance will hel]) to 

 make the electrifications great, and so there will be a better chance of 

 our being able to produce a spark. 



We may weaken the strength of this air-gap by reducing the pressure 

 of the au' in it. To do this the ends of the conductors, or wires con- 

 nected witli them, must lead into an exhausted air vessel, such as a 

 Geissler's tube. There is no donbt that much longer spaiks may thus 

 be produced, but they are so dim and diifused that when dealing with 

 very minute quantities of electricity those K})arks in a vacuum are not 

 more easily seen than the smaller and intenscr s[)arks in air at atmos 

 l)heric pressure. The additional complication and difficulty of manii)u- 

 lation from having the terminals in a. vacuum are not compensated for 

 by any advantages. This whole dete(;ting ap])aratus works on some- 

 what the same i)rinciple as a resonator of definite size connected with 

 one's ear when used to detect a feeble note of the same pitch as the 

 resonator. Such a resonator might very well be used to find out where 

 this note existed and where it did not. It wotdd detect where there 

 were compressions and rarefactions of the air i)rodncing currents of air 

 into and out of your ear. In the same way the conductor sparking tells 

 where there are alternating electric forces making currents alternately 

 up and down tlie conductor, and ultimately ele(;tri}ying the end enough 

 to make it si)ark. In the sound resonator there is nothing exactly like 

 this last phenomenon. We have much more delicate ways of detecting 

 the currents of air than by making them break anything. If anybody 

 would allow the electric currents from a Hertzian detector to be led di- 

 rectly into the retina of his eye, it would probably be a very delicate 

 way of observing, though even in this direct application of the cuirent 

 to an organ of sense it is possible that these very rapidly altctnating 

 currents miglit fail to produce any sensible effect, for they are not 

 rapid enough to pi-oduce the ])hoto-chemical effects by which we see. 



To recapitulate the arrangements proposed in order to detect whether 

 electric force is propagated with a finite velocity, and if possible to 

 measure it if fiuite, it is proi)osed to create electric oscillations of very 

 great rapidity, oscillating some four or five bunded million times per 



