raised to balance the unknown voltage.
The balance point is detected by
means of a galvanometer in series with the two opposed EMFs.
c. Thus, the potentiometer stability and linearity depend on a stable
fixed resistor, a stable CCS source and a transformer turns ratio.
It is
therefore inherently more accurate than conventional potentiometers which
rely on the stability and linearity of variable resistors.
d. The secondary current is set by the combination of a tracking
amplifier and a slave amplifier.
The tracking amplifier monitors the
voltage across the primary turns ratio times the primary current.
Fine
adjustment is provided by the slave amplifier which varies the current until
the flux in the current comparator transformer is zero (i.e., the ampere-
turns in the primary and secondary windings exactly cancel). The zero flux
condition is sensed by two high permeability cores mounted within the shield
of the toroidal transformer. The windings on these cores are connected in
series opposition and the cores are driven into saturation by a square wave
oscillator. Any dc flux in the toroidal transformer core causes one sensing
core to saturate slightly before the other and results in a modification of
the output voltage pulses at the junction of the two windings.
A peak
detector circuit converts these voltage pulses at the junction of the two
windings. A peak detector circuit converts these voltage pulses into a dc
signal whose magnitude and polarity are proportional to the magnitude and
polarity of the flux in the toroidal transformer core. This dc signal then
zero.
7.
Factors contributing to accuracy.
a. GA ZERO. Any thermal EMFs in the measuring circuit will appear in
the output of the potentiometer. These spurious EMFs are compensated for by
the GA ZERO control which introduces a current in the secondary winding and
produces an equal and opposite EMF across the generating resistor.
The
current is introduced in such a manner that it does not interfere with the
ampere-turns balance of the current comparator. As the net thermal EMF will
be of one polarity, the indication that it has been exactly cancelled is
that the galvanometer reading does not change when the currents in the
potentiometer are reversed.
The GA ZERO control is adjusted until this
condition prevails.
b. Generating resistor. The generating resistor is designed to have an
extremely low power coefficient so that it makes only a small contribution
to nonlinearity as the secondary current is increased from 0 to 200 mA.
c. Fractional turns. The effect of fractional turns for the fourth dial
is produced by a resistive divider which diverts l/l0th of the primary
current to a switched 10-turn winding. Each step of the fourth dial switches
1/l0th of the current produced by each step of the third dial. The accuracy
required of this 10:1 current division is only 1 part in 103 of the accuracy
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