(7) ZERO pushbutton.
When this button is pressed, the CCS is
disconnected from the primary winding (i.e., zero ampere-turns) and the
detector terminals are connected across the voltage-generating resistor.
Thermal EMFs can then be balanced out by using the GA ZERO control.
Two
dots are engraved on this pushbutton. When these dots are at right angles
to the line of pushbuttons, the ZERO, STD, and MEAS pushbuttons will remain
depressed until another is selected.
When the dots are parallel to the
buttons, all the buttons are spring return, and cancel out automatically
when released.
(8) STD pushbutton.
This button is pressed when it is required to
standardize the potentiometer.
(9) MEAS pushbutton.
Pressing this pushbutton connects the
potentiometer output to the EMF terminals and allows measurement of the
unknown external voltage.
b. Terminals and connectors include the following:
(1) Ground.
A chassis ground terminal is located on the front panel.
(2) +
EMF
-
The
voltage
to
be
measured
is
connected
to
these
terminals.
(3) + SC - These terminals are connected to a standard cell when
standardizing the instrument.
(4) + DET - The galvanometer system is connected to these terminals.
(5) + CCS These terminals are connected to the CCS.
(6) Fuse.
A
2A
fuse
is
mounted
on
the
front
panel
of
the
potentiometer.
(7) Line
cord.
The
line
cord
is
connected
underneath
the
potentiometer.
(8) Readout socket. A multipin socket which mates with the connector
from a Readout Display unit mounted underneath the potentiometer.
c. ZERO adjustment. Operation of the ZERO button connects the external
detector across the voltage-generating resistor.
It also disconnects the
CCS from the primary winding and simultaneously reduces the slave power
supply to zero current, thus producing nominally zero voltage across the
generating resistor.
Any thermal EMFs, however, will cause an output
voltage which will be detected by the galvanometer.
The effect of these
thermal EMFs is eliminated by injecting a small current through the
generating resistor to produce an equal and opposite EMF.
The complete
procedure would be carried out as follows:
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