Some meters must be constructed with a high degree of
sensitivity. The sensitivity is determined by the amount of current required to
produce a full-scale deflection of the indicating needle. Very sensitive
movements may require as little as 0.00005 amp to produce a full-scale deflection.

This value is commonly called 20,000 ohms per volt, because
it requires 20,000 ohms to limit the current to 0.00005 amp when an emf of 1
volt is applied. Movements having a sensitivity of 1,000 ohms per volt are
commonly used by electricians when the power consumed by the instrument is of
no consequence.

In electronic work, where very small currents and voltages
must be measured, instruments of very high sensitivity are required. Electronic
measuring instruments, such as the vacuum-tube voltmeter (vtvm) or the
solidstate voltmeter (ssvm), are normally used for the measurement of currents
and voltages in electronic circuits.

These instruments are designed to isolate the measuring
circuit from the circuit being measured, hence very little loading is applied
to the circuit being measured. To understand the importance of sensitivity in
an instrument for testing certain values where current flow is very small, it
is well to consider a specific example.

A 100-volt battery is connected across two resistors in
series. Each resistor has a value of 100,000 ohms, making the total resistance
of the circuit 200,000 ohms. Since the two resistors are equal in value, it is
obvious that the voltage across each will be 50 volts.

If we wish to test this voltage by means of a voltmeter
which has a 1,000-ohms-per-volt sensitivity, we will discover that a large
error is introduced into the reading.

Assume that the voltmeter has a range of 100 volts and that
it is connected across R, between the points A and B. Since the voltmeter has a
sensitivity of 1,000 ohms per volt, its total resistance will be 100,000 ohms.
When this is connected in parallel with R,, the resistance of the parallel
combination becomes 50,000 ohms, and the total resistance of the circuit is now
150,000 instead of 200,000 ohms.

With the resistants;, between A and B 50,000 ohms and the
resistance between B and C 100,000 ohms, the voltage drop will be 33.3 volts
between A and B and 66.7 volts between B and C. It is apparent then that the
voltmeter used would not be satisfactory for this test.

If we connect a voltmeter with 20,000 ohms-pervolt
sensitivity across R,, we will obtain a much more accurate indication of the
operating voltage. i The voltmeter has an internal resistance of 2,000.000 I
ohms, and this resistance, combined in parallel with R,, will produce a
resistance of 95,238 ohms.

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