The 1422 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 an AC bridge measurements, either as
a built-in 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.
Two-terminal - The 1422-D is
a dial-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 alloy 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
on 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 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), in 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.
 
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