9.
If a circuit is composed of two dissimilar metals, a net voltage will
result if the two dissimilar junctions are maintained at different
temperatures.
These thermoelectric voltages, also known as thermals,
thermo-couple voltages, or Seebeck voltages, can be reduced by using metals
having low thermoelectric potentials, and keeping all junctions at the same
temperature.
The terminals of the Model 845AR are made of pure copper,
gold-flashed to prevent tarnish.
For lowest thermal voltages, all
connections to the Model 845AR should be made with pure copper wire.
Silver-plated copper or solder-coated copper also produce satisfactory
results. Tinned copper is less satisfactory than silver-plated or copper-
coated copper.
Nickel and nickel-based alloys are not suitable for
connections to the instrument.
Excellent results can be obtained using
ordinary TV twin lead, or even lamp cord if high insulation resistance is
not required. If shielding is necessary, use a length of flat braid over
the cable.
10. Due to the very high input resistance and extreme sensitivity of the
Model 845AR, it is charge sensitive.
Thus, a person's body potential, an
electrostatic voltage, can cause charge redistribution at the input to the
instrument and result in meter needle deflection as a hand approaches the
input terminals. Careful shielding will eliminate this problem. Also, due
to charges that may be deposited on the input terminals when the OPR-ZERO
switch is set to ZERO, an appreciable transient will result when the switch
is set to OPR if nothing is connected to the input terminals. Turning the
switch back and forth will dissipate this charge, eliminating the problem.
With a high source impedances, the response of the instrument is unavoidably
slow due to the low pass filter used to suppress superimposed noise.
However, the design of the low pass filter is such that common mode
rejection is extremely high while the response time for the normally
encountered low source impedances is very fast.
11. The instrument is designed to withstand up to 1100 volts dc or 1100
volts peak ac continuously applied between any two of the three input
terminals or between cabinet ground and any of the three input terminals
regardless of the setting of the RANGE or OPR-ZERO switch.
However,
repeated or continuous overloads above 200 volts in the ranges below 3
millivolts will result in dissipation in protective, low-pass-filter
resistor R110. This will result in thermal voltages which may take several
minutes to subside after the overload is removed.
12. The instrument has an inner chassis connected to the GUARD terminal on
the front panel. Ordinarily, this GUARD terminal is strapped to the COMMON
terminal. When connected in this way the inner chassis serves as a shield.
This greatly improves the leakage resistance to ground and the common mode
rejection.
However, since the inner chassis is available at the GUARD
terminal, it may be driven at the same voltage as the COMMON terminal. This
further increases the leakage resistance and common mode rejection by about
ten times. The voltage used to drive the GUARD terminal should be obtained
from a separate source or by means of a voltage divider connected directly
across the source so that the leakage currents do not cause voltage drops
across impedances in the circuit under measurement.
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