ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 257 



a. This experiment illustrates the phenomenon of electric reso- 

 nance. Each oscillation of the circuit in Fig. 187 induces a 

 slight electromotive force in the circuit of Fig. 1 88, these succes- 



s 



to electric 



machine 



to electric 



machine 



wire 



rod 



w 



w wire 



Fig. 187. 



1 



rod 



wire 



rod 



I? 



=0 



w wire 



Fig. 188. 



IV 



rod 



sive electromotive forces are in unison with the proper period of 

 oscillation of the circuit in Fig. 188, and therefore their effect is 

 cumulative. 



The Tesla induction coil is usually arranged so that the induc- 

 tance of its primary circuit can be adjusted, thus altering the 

 frequency of the oscillatory discharges through the primary coil. 

 Then by adjusting the inductance of the primary until the fre- 

 quency of oscillation of the primary circuit is the same as the 

 frequency of oscillation of the secondary circuit, the successive 

 surges of current in the primary coil become cumulative in their 

 effect on the secondary (resonance). 



144. Water waves in a canal. Before attempting to describe 

 electrical waves, it is desirable to consider some of the phe- 

 nomena presented by water waves. A water wave consists of a 

 moving hill of water, a given particle of water is set in motion 

 when the wave reaches it, and comes immediately to rest after 

 the wave has passed. What supports the hill of water, and 

 what produces the unbalanced force which causes the water to 

 18 



