595 
25 
Thus at first the charge 
is produced and thrown on the capac- 
itor plates. Then as the pressure 
diminishes the capacitor discharges 
into the crystal and through the re- 
sistance R. It is apparent that if 
the resistance were infinite the 
voltage on the capacitor would follow 
Voltage 
Vo (observed) 
the original pressure faithfully. 
Figure 15 - Distortion of a 
However, the presence of a finite re- Piezoelectric Gage Signal 
sistance causes a leakage through that Due to RC Leakage 
Vv, is the value which would be obtained if the 
as well as back into the crystal. Thus eeea “Ua Sees. 
the voltage across the capacitor is 
always less than it would otherwise be. In fact, even if the original pres- 
sure were always positive, it would be possible, because of the leakage 
through R, for the capacitor to reverse its charge and indicate an apparent 
negative force. The distortion of the signal from a piezoelectric gage due 
to leakage is shown in Figure 15, in which V>) is the observed voltage and V, 
is the voltage to be expected for zero leakage. 
It is apparent that the relative error (V, - V,)/V, increases as 
time progresses. Thus if it is desired to study the late phases of the rec- 
ord, it is necessary to make the product RC quite large. This requirement is 
more severe than if the peak value alone were being investigated. An analysis 
of the distortion is given below. 
Let i, = -dq/dt be the total current flowing out of the capacitor, 
i, = -dQ/dt be the current flowing into the crystal, and i, = g/RCbe the 
current flowing through the resistance R. Then 
Wen te Ue [12] 
or 
ee a 
dt RC dt 
and 
Cl ie eee 
dt RC dt 
q(t) = ABUL [{22 _ Serine dr + e| 
t 
= oT O¢4) oR Dh f(r) e dr + c| 
