Low Frequency and their Resonance. 333 



variation in the size of the sparks if the adjustment is properly 

 done. I have used as much as four storage cells in series as 

 my exciter and have no doubt that with a properly constructed 

 vibrator as much as 100 volts could be used without any an- 

 noyance arising from the sparks. But the interrupter must 

 be kept in a place entirely free from jars or vibrations. For 

 even with the storage battery just mentioned, when the voltage 

 is only 8 volts, vibrations of the floor or table or singing a 

 note which is nearly in unison with the vibrator will disturb 

 its vibrations sufficiently to cause a dissonance between the 

 vibrator and the circuits. This dissonance manifests itself at 

 once by lively sparking which subsides as soon as the dis- 

 turbance ceases. It is this very state of extreme sensitiveness 

 of the electrically tuned up system which makes the work 

 with the vibrator exceedingly instructive and interesting. 



The secondary coil afty (fig. 4) supplies the alternating cur- 

 rent for the bridge. The number of its turns is small in com- 

 parison to the number of turns in the primary and the current 

 in it is also small in comparison with the current in the pri- 

 mary, on account of comparatively high resistance in the bridge, 

 so that the variation of the secondary current does not inter- 

 fere with the established resonance in the primary. We can 

 therefore tune this circuit without disturbing the adjustment 

 of the primary. The tuning is performed by means of the 

 auxiliary coil I, the condenser M, and a telephone placed in 

 shunt with a part of the circuit whose resistance is high enough 

 to give sufficiently intense sound in the telephone. 



Finally, the bridge itself containing the telephone T is tuned 

 by means of the auxiliary coil K and condenser L. This part 

 of the tuning does not interfere with any of the previous ad- 

 justments on account of the extremely small value of the cur- 

 rent which has to pass through the telephone to make the 

 sound in it sufficiently intense. 



It is needless to observe that the upper harmonics which the 

 vibrator tends to produce are completely wiped out in the tele- 

 phone circuit and that, therefore, with this arrangement we 

 can employ the zero method of measurement, using as out de- 

 tector the ordinary Bell telephone. 



The second kind of apparatus that can be employed is a coil 

 in series with a condenser as in fig. 5. A spark space is inserted 

 in the circuit between the coil and the condenser and the ex- 

 tremities of this spark space are connected to the poles of an 

 influence machine, or a high Yoltage electromagnetic gener- 

 ator, and an air blast is applied to the spark space when the 

 sparks begin to pass. An alternating current of any frequency 

 may thus be generated in this coil-condenser circuit by a proper 

 adjustment of capacity and self-induction and by gradual trans- 



