HEKTZ'S KXl'ERIMENTS. 213 



(.'>) Tlu' third i)r(»i)l<Mii is: How can we expect to detect wliere there 

 are loops and wiiere tiieic are nodes .' Recall the eflects of electric force. 

 It tends to move electrilied bodies. If then an electrified body were 

 placed in a loop it would tend to vibrate ui> and down. This method 

 may i)ossibly be employed at some future tinu\ and it may be part of 

 the cause of photographic actions, for these have recently been conclu- 

 sively proved to l)e due to electric tbrce; but the alternations of electric 

 force from positive to nepitive that have to be emi)loyed are so rapid 

 that no body large enougli to be easily visible and electrified to a^ reason- 

 able extent could be expected to move sutticiently to be visibly dis- 

 turbed. It is possible that we luay tind some way of detecting the 

 vibrations hereby given to the electritied ions in an electrolyte; and 

 it has recently been stated that waves originated electrically shake the 

 elements in sensitive photographic films sufficiently to cause changes 

 that can be develo])ed. The other action of electric force is to produce 

 an electric current in a conductor and a resultant electrification of the 

 conductor. Two effects due to this action have actually been used to 

 detect the existence of the wave of electric force sent out by a body 

 alternately electrified positively and negatively. One of these is the 

 heating of the conductor by the current. Several experimenters have 

 directly or indirectly used this way of detecting the electric force. The 

 other way, which has proved so far the most sensitive of all, has been 

 to use the electrification of the conductor to cause a spark across an 

 air s]>ace. Tliis is the method Hertz originally emph)yed. A pnori^ 

 one would not have expected it to he a delicate method at all. It takes 

 very considerable electric forces to produce visible sparks. On the 

 other hand, the tinu' the force need last in order to ])roduce a spark is 

 something very small indeed, and hitherto it has not been possible to 

 keep up the alt<'ruate electrifications for more than a minute fraction of 

 a second, and this is the reason why other api)arently more promising 

 metiiods have failed to be as sensitive as the nu4hod of [»roducing 

 s])arks. If two conductors be placed very close to one another in 

 such a direction that the electric force is in the line joining them, their 

 near surfaces will be op])ositely electrified when the electric force 

 acts on them, and we may expect that, if the force be great enough 

 and the surfaces near enough, an electric spark will i)ass I'rom one to 

 the other. This is roughly the arrangement used by Hertz to detect 

 whether there are loops and nodes between the originator of the waves 

 and the reflector. 



Now aiises the problem of how to electrify tlie body alternately i)osi- 

 tivcly and negatively with sufficient rapidity. How rapid is '• with suffi- 

 cient rai)idity'if" To answer tiiis we nuist form someestimate of how raj)- 

 idly we may expect the waves to be propagated. According to Maxwi'Ifs 

 theory they should go at the same rate as light, some 3tH),()(H), (»()(» of 

 meters per second, and it is evident that if we are going to test Maxwell's 

 theory w«^ must mak<* provision f()r sufficiently rai)id electric \iliratious 



