Sensitivity of Voltmeter :
The sensitivity of a voltmeter is defined as the reciprocal or inverse of the full-scale deflection current (Ifsd) of the basic movement. It is denoted by the symbol S and expressed in Ω/V.
Where Ifsd is the amount of current required to deflect the pointer of the basic meter to its full-scale position.
Voltmeter sensitivity is also known as ohms-per-volt rating of the voltmeter. It can also be expressed as the ratio of the total resistance of the circuit Rt to the voltage range V of the voltmeter.
Where,- Rm = Internal resistance of movement
- Rs = Multiplier resistance
The above expression of voltmeter sensitivity can be used to determine the resistance of the multiplier resistor used in a voltmeter circuit to extend the range of the voltmeter.
Loading Effect of Voltmeter :
The loading effect of a dc voltmeter refers to the phenomenon in which a negative error is produced in the voltmeter reading (measured voltage), due to the low internal resistance (i.e., low sensitivity of the voltmeter).
Sensitivity is the main factor in selecting the voltmeters for measuring the voltages of the desired range. If the sensitivity of the voltmeter is low, then the voltmeter gives accurate readings for a low resistance circuit and inaccurate and unreliable readings for a high resistance circuit, which is known as loading effect.
Let us consider two circuits, in which voltmeter connected across the resistance.
In the figure shown above, the resistance of the voltmeter is considered to be 50 kΩ and the resistance across which voltage is to be measured is low compared to the resistance of the voltmeter i.e., 500 Ω. As current flows through the path of low resistance, maximum of the current flows through the low resistance and only a part of the current flows through the resistance of the voltmeter. Hence, the voltmeter gives the true value of the reading.
In the figure shown above, the resistance of the voltmeter is 50 kΩ and the resistance across which the voltage is to be measured is very high i.e., 8O kΩ. In this circuit, most of the current flows through the voltmeter, and less current flows through the resistance across which the voltage has to be measured.
This is because the current always chooses a low resistance path (voltmeter path). Due to this the voltage drop across the resistor will be less when compared to the actual voltage drop before the voltmeter has connected. Hence the voltmeter shows a value that is lower than the true value of the reading.
From the above, if a voltmeter having a low sensitivity is used to measure the voltage,- For low resistance circuits, it provides correct readings.
- For high resistance circuits, the voltmeter acts as a shunt for that portion of the circuit, and hence, the equivalent resistance of that portion decreases. As a result, the voltmeter indicates a voltage value lower than the actual voltage. This effect is known as the loading effect of the voltmeter.
To avoid the loading effect, a voltmeter of high sensitivity should be used or it can be eliminated to some extent by using a voltmeter with a very high resistance when compared to that of the resistance in the circuit.