DETERMINATION OF WAVE FORM 

 The phase displacement of any particular harmonic is given by 



If No and N represent the number of vibrations per 

 corresponding to T a and T respectively, 



N -* 



T N 6,000 



If ~N : "60" = 100 ' then 



tan $\ = __ = 0.020 o 1.15 of the fundamental 



OOQ __ 



1 800 



tan P' 9 = 10 000 - 81 = ' 182 10 ' 3 of the ninth harmonic 

 tan 0' 21 = 0.439 o 23.7 of the twenty-first 



harmonic. ' 



If the length of one complete cycle of the curve is 2.5 in., the 

 corresponding linear displacements are 



for /S'l ........... 0.0080 in. 



for /3' 9 ........... 0.0079 in. 



for 0' 21 .......... 0.0078 in. 



It will be noted that all the components are shifted from 

 the positions they would occupy with a perfect instrument by 

 practically the same amount, so that no error of importance is 

 introduced by the phase displacements. 



The Electrostatic Oscillograph. 6 The electrostatic oscillo- 

 graph is a form of electrometer so designed that the rate of 

 free vibration is high and the damping of proper value. The 

 instrument is used heterostatically. The high free period 

 necessitates a strong controlling force, and this, together with 

 the fact that electrostatic forces are small when low potentials 

 are concerned, means that the electrostatic oscillograph is natu- 

 rally best adapted to high-voltage work. For such work it 

 possesses the advantage that it consumes no energy and that the 

 current required is exceedingly small, being only that necessary 

 11 



