fed to the converter unit is developed across a resistor (R-1) in the junction box, Figure 15, which is in series with these meters. Identification markers may be injected during a survey by the oircuit consisting of battery B-1, potentiometer R-9 and timer T-1l. The input from the detector is disconnected and a voltage is supplied to the grid of the balanced amplifier to drive the output signal to full scale. The marker is initiated by an external push button. Its duration is determined by timer T-1 and its amplitude by R-9. 30202 Converter The converter section turns the D.C. voltage developed across R-1 into amplitude modulated pulses. A small part of the full-wave rectified 60 cps line voltage appears across R-25 end is amplified by V-2B. The output of V-2B triggers a univibrator producingconstant amplitude square waves (Figure lja) at a 120 pps rate, converter tube V-2A. These are fed to the grid of the upper The signal from the bridge circuit is fed into ‘the grid of tube V-3, which is in series with the cathode of V-2A. The detector signal determines the current through both V-3 and V-2A and hence the gain of V-2A. Therefore, the amplitude of the pulses appearing across R-1]1 (Fig. lb) are proportional to the signal from the bridge. The pulses across R-11 are transformer coupled to the comparator unit, Batteries, B-2 and 43, and resistors, R-12, 13 and 14, fix the bias of V-3 so that the tube operates over a linear portion of its characteristic. A floating ground is used since the signal from the bridge is at a potential above chassis ground. R-26 and C-8, 9 and 10 provide a filter from the floating ground bus to chassis ground. - 30 -