b. To use the multiplication factor to determine the actual input
voltage, first assume that 60 volts are applied to the .75-volt binding post
and the potentiometer is monitoring the 1.5-volt terminal.
As mentioned
above, the multiplication factor must be determined.
To do this, 75 is
divided by 1.5 and a multiplication factor of 50 is obtained.
Next, the
approximate input voltage (60 volts) is divided by the multiplication factor
(50). This gives 1.20 volts which will be preset on the potentiometer. The
measured voltage is then multiplied by the factor of 50 to determine the EMF
present at the 75-volt terminal, or,
input to volt box = multiplication factor x measured EMF
Learning Event 6:
DESCRIBE CARE AND PRECAUTIONS WHEN USING THE POTENTIOMETER
1.
The first trials for balance should check the action of the protective
resistance in both the galvanometer and the standard cell circuits to
prevent injury from excessive currents.
a. Care must be exercised to minimize the current passing through the
standard cell, since it is essentially an open-circuit device and its EMF is
altered by polarization resulting from current flow.
b. The key should be depressed only momentarily to note the direction
of needle movement; it should never be left closed.
c. The maximum cell current should be limited to a value below 10
microamperes, preferably to a fraction of a microampere.
2.
To avoid short-circuiting the standard cell, it is best to connect it
to the potentiometer carefully after all other connections have been made
and to disconnect it first on dismantling the hookup. Careful checks should
also be made of the polarity when connecting the standard cell and the
working cell to the potentiometer. It may further be necessary to maintain
the standard cell at a constant temperature and to check its EMF
periodically.
3.
False deflection of the galvanometer during damp weather measurements
may require guard circuitry, for example, mounting the potentiometer
equipment on insulating blocks, each of which rests on a metal electrode,
and connecting the electrodes to the positive of the working battery or the
highest external potential to which the potentiometer is connected.
a. Additional guard circuits may be necessary, if not included in the
potentiometer, for prevention of stray currents at the detector binding
posts, the galvonometer key, and the external voltage connections.
b. The
galvanometer
circuit
should
be
protected
by
a
high
series
8
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