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The 1422 Variable Air Capacitor.
Variable Air Capacitor is a stable and precise variable air capacitor intended for use as a continuously adjustable standard of capacitance.
One of the most important applications is in AC bridge measurements, either as a built-in standard or as an external standard for substitution measurements.
It is available in a variety of ranges, terminal configurations, and scale arrangements to permit selection of precisely the required characteristics.

1422 Air Variable Capacitor

Model  Description 
1422  1422 Capacitor Variable. Air Capacitance.

Stability: better than 0.02% full scale per year
Settable to 40ppm
Low Temperature coefficient, low losses
Wide selection to suit needs
3 different models Laboratory Standard
Working Standard
Calibrating Working Standards
Capacitance Measurement Functions
Substitution Measurements
Calibrating Instrumentation

Two-terminal - The 1422-D is a dual-range, two-terminal capacitor, direct reading in total capacitance at the terminals.

Three-terminal - The 1422-CB and -CL are three-terminal capacitors with shielded coaxial terminals for use in three-terminal measurements. The calibrated direct capacitance is independent of terminal capacitance to ground, and losses are very low.
The 1422-CL has particularly low, constant terminal capacitance, 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 (Types 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 sight residual eccentricities of the worm drive and requires interpolation over only short intervals. This precision calibration is available for the 1422-CL model. A plastic enclosed certificate of calibration is supplied, giving corrections to one more figure than the tabulated accuracy.

1422 Air Capacitor Specification
1422 Precision Capacitors Two Terminal 1422-D Three-Terminal
1422-CB
Three-Terminal
1422-CL

Capacitance Range (Min/Max)
100/1150pF 35/115
50/1100pF 10/110pF


Scale, pF/Division

0.2                  0.02

0.2

0.02

Initial Accuracy: ± Picofarads
Direct-Reading (Adjustment):
Total Capacitance

1.5*                 0.3*

1.5

0.1

With Corrections from
Calibration Chart (supplied)
Total Capacitance

n/a                   n/a

n/a

0.04

With Corrections from
Precision Calibration (extra charge)
Total Capacitance

n/a                   n/a

n/a

0.01

Residuals (typical values):
Series Inductance, µH 0.06
Series Resistance, V at 1MHz 0.04


           0.10
           0.05

0.14
0.1


0.13
0.1

Terminal Capacitance, pF, typical
high terminal to case
low terminal to case


min/max scale
min/max scale

36/35
58/53
34/33
58/55