218 hertz's experiments. 



variety of circumstances to aecuiiiulate the effect of a series of im- 

 pulses, aud is avoided in auotlier immense variety of circumstances 

 to prevent accumulating the effect of a series of impulses. We see, 

 we liear, we pliotogTaph by using it; we use it to make musical sounds, 

 to keep clocks and watches going, to work telegraplis. By avoiding 

 it carriages drive safely over rough roads, shi^is navigate the seas, 

 the tides do not now overwhelm the hiiid, the earth and planets 

 preserve their courses round the sun, aud the solar system is saved 

 from destruction. Resonance maybe thus described: If a system is 

 able to vibrate by itself in any way, and if we give it a series of im- 

 pulses, each tending to increase the vibration, the effect will be cumu- 

 lative, and the vibration will increase. To do this the im^iulses must 

 be well timed, at intervals the same as the i^eriod of vibration of the 

 system itself. Otherwise some of the impulses will tend to stop the 

 the vibration, and only some to increase it, and on the whole the effect 

 will be small. 



In order to use resonance in the construction of the detector of 

 waves of electric force, we must make our detector so as to be capa- 

 ble of an electric vibration of the same period as the generator of the 

 waves. If we do this we may expect the currents i)roduced in it to 

 be increased by each wave, and thus the electrification at its ends to 

 increase, and so increase tlie chance of our being able to produce a 

 visible spark. Tw^o ways of using a detecti)r have been mentioned. 

 One is to observe the heating of a conductor by tlie current in it, 

 and the other to observe a spark due to the electrification at the 

 end of the conductor. The latter is the most sensitive and has 

 been most frequently employed, aud is the nietliod first employed by 

 Hertz. Two forms of detector may be used for observing sparks. One 

 form consists of a single conductor bent into a circle with its two ex- 

 tremities very close together. An electric charge can oscillate from 

 one end of this to the other round the circle and back again. If the 

 circle be the proper size, about 70^'" in diameter for the large-sized 

 oscilhitor and about 8"" in diameter for the smaller-sized one de- 

 scribed in the last article, the period of oscilhition of this cliarge will 

 be the same as that of the charge on the generator of the waves, and 

 its oscillation will be increased 'by resonance until, if the ends of the 

 circular wire be close enough together, the opposite electrification of 

 the ends will become great enough to cause a spark across the gap. 

 The other form of detector depends on using two conductors, each of 

 which has the same period of electric oscillation as the oscillations we 

 wish to detect. These are placed in such a position that an end of one 

 is near that end of the other which will at any time be oppositely electri- 

 fied. For example, if the electric force in our waves be in vertical lines, 

 then if we place two elongated conductors, one vertically above the 

 other and separated by a very small air space, the electric force alter- 

 nating up and down will cause currents to run up and down the con- 



