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.
