The Type 126 General Radio Capacitance Bridge at the left is in the Greenslade Collection. It was designed to measure capacitances as low as a few picofarads to a precision of at least 0.1%,
This is an alternating current bridge with the standard diamond pattern of the Wheatstone bridge. The two upper arms are resistances, while the two lower arms are capacitances. One is the unknown, and one is an [external] calibrated capacitor. An external 1000 Hz oscillator, such as a General Radio Type 213 Audio Oscillator, is used to drive the bridge. The detector is a pair of high-impendance magnetic earphones, driven through a coupling transformer to increase the net impedance of the detector to the rest of the circuit. This is necessary because at 1000 Hz a 1000 pf capacitor has an impedance of 160,000 ohms.
The cabinet is lined with sheet copper, and the solder connections are up to the usual high General Radio standard.