DETERMINATION OF WAVE FOKM 633 



each other, so that it is not necessary that the vibrators be keot 



approximately at the same potential. With A, the potential 



ference between the two vibrators and also that between the 



'ibrators and the frame of the instrument should not exceed 



50 volts. 



The minimum free period which it is practicable to give this 

 form of vibrator appears to be about Ko.ooo second. To attain 



B 



Tension Adjustment 



FIG. 397. Oscillograph vibrators. A, Cambridge Scientific Instrument Co. ; 

 B, General Electric Co. 



this figure the parts, especially the mirror, must be exceedingly 

 delicate; this increases the liability to accident and the difficulty 

 of making repairs. 



Instruments with such a high rate of vibration are useful and 

 indeed essential in certain research work, but they must be 

 used by skilled operators. For general engineering work it is 

 better to be content with a more robust vibrator, having a free 

 period of about J,ooo second. Such an instrument will follow 

 the waves usually encountered, closely enough for practical 

 purposes. 



