hertz's experiments. 221] 



the electric force were ino])aiiate(l iiistiuitniieonsly there iiiij^ht be one 

 ])hice wliere tlie action of tlie currents induced in our rellcctiny- sheet 

 m'utralized tlie direci action of our <ienerator. but tliere could not be a 

 series of two or more such places between the iu'enerator and the re- 

 flecting' sheet. That there are more than one proves that electric force 

 is propagated from place to i)lace, and does not oc<'ur siinultaneonsly 

 everywhei'e. It sets the crowning stone (ui ^MaxwidTs theory that elec- 

 tric force is due to a m.cdiuni. Witliont a medium tliere can be no 

 ])ropagation from })lace to place in time. It only lemainsto contirmby 

 calculation that the rate of ])ropagation is th<' same ;(S that of light. 

 This is a complicated matter. It involves the (piestion of how fast 

 should, on any theory, the charge oscillate n}) a.nd down a conductor. 

 The problem has only been accurately solved in a few six'cial cases, 

 such as that of a sphere by itself. The conductors tliat have been 

 em])loyed are not this shape, are not by themselves, and- so only rough. 

 api)roximations are possible as to the rate at which these oscillations 

 occur. Knowing the wave lengtli will not <leterniine the velocity of 

 l)roi)agation unless we know the period of vibration : and consiMjuently 

 this direct measure of the velocity has only been rouglily nnide; but 

 it agrees as accurately as could be cxpecti'd with Maxwell's theory 

 that it must be the same as the \-clocity of light if electri(;al i)henomena 

 are due to the same medium as light. Tlie conviction tliat more accu- 

 rate (h'terminations will conlirm this agreement is founded niton safe 

 ground. 



It was i)oiiitc(l out tliat the a'ther that transmits light and is set in 

 vibration by the molecules of matter j-an hardly avoid moving them 

 itself. This a;tlier can hardly help lia\ing other properties than 

 merely transmitting a compnrativ<'ly small range ol' vibrations. It can 

 hardly hel]) inoducing other ])henomena. \\'hen it has been shown 

 that, if there is a medium concerned in conv<'ying electric and mag- 

 netic actions, it must possess proiierties which would enable it to 

 transmit waves like light; and when it has been sliown that there is a 

 medium concerned in con\eying electric and magnetic actions, and that 

 the rate at which they are conveyed is approximately the same as the 

 rate at which light is i»roi)agated; the conclusion is almost unavoid- 

 able that we ai'c dealing with the same medium in both cases, and that 

 futuri^ experiments, cai)able of accurate calculation and observation, 

 will confirni the conclusion that electric* force is ])ropagated through, 

 and by means of, the luminiferous a'ther with the velocity of light. 

 We really know \ery little about the natuic of a wave of light, ^\'e 

 know a great <leal more about electric and magnetic forces, and much 

 may be tearnt as to the nature of a wave of light by studying it under 

 the form of a wa\e of electric force. The waves ))roduced by the 

 Hertzian generator may be a nu'ter long or more. The dithculty is to 

 get them short enough. We know a good <leal about how they are 

 prodiUH'd, and from this, and also by means of suitable detectors, we 



