DETERMINATION OF WAVE FORM 



the voltage between F x and F 2 at any instant and let B be the 

 constant e.m.f. of the well -insulated battery. FI, F2 and i 

 may be regarded as forming the elements of one electrometer 

 and FI, F 2 and s 2 , those of another. The two moving elements 

 or needles are mechanically coupled by the mirror, ra, which is 

 cemented to both. 



It has been shown (see page 250) that the force acting on the 

 movable element of an electrometer may be represented by 



/ = K(2Vd + d 2 ) 



where d is the fall (or rise) of potential from quadrant 1 to quad- 

 rant 2 and V is the fall (or rise) of potential from quadrant 2 

 to the needle. Applying this to the case in hand, for the electro- 

 meter FI, F 2 and si, 



+KBe 

 and for the electrometer FI, F 2 , s 2 , 



Therefore, the mirror which is cemented between $1 and s 2 is 

 acted upon by a couple proportional to Be. If the natural period 

 of the instrument be high (a frequency of 3,500 vibrations per 

 second may be obtained), and if proper damping is employed, 

 the deflection at every instant is = kBe, that is, the instrument 

 follows the wave as does an ordinary oscillograph. 



Obviously it is necessary to prevent sparking between the 

 various parts of the instrument and across the condensers. 

 Consequently the entire system, FI, i, **, ^2, is immersed in a 

 transparent oil of high dielectric strength and the condensers 

 are similarly treated. It is essential that no dielectric losses 

 occur, as they would introduce phase displacements. 



Adjustment of Electrostatic Oscillograph. If the voltage . 

 is zero, the instrument should experience no deflection; it is 

 however, practically impossible to construct the instrument wit 

 the mathematical accuracy assumed above, so the 

 adjustment is necessary. . 



Make B equal to zero, that is, connect both strips to d ; 

 apply the full alternating voltage between FI and Ft. In gen al, 



