GenRad 1422 Series Variable Air Capacitors

• Range as low as 0.002 pF and as high as 1150 pF (Depending on Model)
• Stability: better than 0.02% full scale per year
• Settable to 40 ppm
• Low temperature coefficient, low losses
• Wide selection to suit Leeds

The GenRad 1422 is a ultra stable and precise variable primary air capacitor intended for use as a continuously adjustable standard of capacitance. One of the most important applications for the 1422 is in ac bridge measurements, either as a built-in standard or as an external standard for substitution measurements.

The 1422 is available in a variety of ranges, terminal configurations, and scale arrangements to permit selection of precisely the required characteristics.

Two-Terminal The GenRad 1422-D is a dual-range, two-terminal capacitor, direct reading in total capacitance at the terminals. For convenience in making substitution measurements, two 1422's have scales reading in capacitance removed, i.e., the capacitance is maximum at the zero reading. These, the 1422-MD and 1422-ME, are also dual-range, two-terminal capacitors.

Three-terminal The GenRad 1422-CB, -CL, and -CD are three terminal capacitors with shielded coaxial terminals for use in three-terminal measurements. The calibrated direct capacitance is independent of terminal capacitances to ground, and losses are very low. The GenRad 1422-CL has particularly low, constant terminal capacitances, making it suitable for measurement circuits in which high capacitance to guard cannot be tolerated.

Construction

The capacitor assembly is mounted in a cast frame for rigidity. This frame and other critical parts are made of aluminum alloys selected to give the strength of brass with the lightness of aluminum. The plates of most models are also aluminum, so that all parts have the same temperature coefficient of linear expansion.  A worm drive is used to obtain high precision of setting. To avoid eccentricity, the shaft and the worm are accurately machined as one piece. The worm and worm wheel are also lapped into each other to improve smoothness. The dial end of the worm shaft runs in a self-aligning ball bearing, while the other end is supported by an adjustable spring mounting, which gives positive longitudinal anchoring to the worm shaft through the use of a pair of sealed, self-lubricating, preloaded ball bearings. Similar pairs of preloaded ball bearings provide positive and invariant axial location for the main or rotor shaft. Electrical connection to the rotor is made by means of a silver-alloy brush bearing on a silver-overlay drum to assure a low-noise electrical contact.

Stator insulation in all models is a cross-linked thermosetting modified polystyrene having low dielectric losses and very high insulation resistance. Rotor insulation,

where used (GenRad 1422-CB and -CL), is grade L-4 steatite, silicone treated.

Accuracy

The errors tabulated in the specifications are possible errors, i.e., the sum of error contributions from setting, adjustment, calibration, interpolation, and

standards. When the capacitor is in its normal position with the panel horizontal, the actual errors are almost always smaller. The accuracy is improved when the readings are corrected using the 12 calibrated values of capacitance given on the correction chart on the capacitor panel and interpolating linearly between calibrated points. Even better accuracy can be obtained from a precision calibration of approximately 100 points on the capacitor dial, which permits correction for slight residual eccentricities of the worm drive and requires interpolation over only short intervals. This precision calibration is available for all models at an extra charge. A plastic-enclosed certificate of calibration is supplied, giving corrections to one more figure than the tabulated accuracy.

Specification

1422 Variable Air Capacitor Basic Specifications

(See Datasheet for Full Specifications)

Stability: Capacitance change with time <1 scale division (0.02% of full scale) per year. Long-term accuracy can be estimated from the stability and the initial accuracy.

Calibration: Measured values (supplied) are obtained by comparison at 1 kHz, with working standards whose absolute values are known to an accuracy of (0.01% + 0.0001 pF). Each comparison is made to a precision better than 0.01%.