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.