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What are Loading Effects of an Instrument

Written by Harpreet

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Loading effects of an Instrument are the alternations that are caused in the circuit conditions such as voltage, current etc. when the instrument is introduced in the circuit for the purpose of measurement. In simple terms, loading effects of an instrument ends up distorting the signal they are supposed to measure. The instrument therefore reads the altered value of the quantity and thus an erroneous measurement is resulted.These loading effects can be better explained by the following two examples:-


Let a voltage has to be measured across a resistance R in the circuit. For this purpose, a voltmeter V is connected across the resistor R in parallel. We know that a voltmeter has a very high resistance value. But since this value is finite, a fraction of the total current passing through R will pass through the Voltmeter V. This will lead to power dissipation in the Voltmeter. The voltmeter extracts this power out of the circuit and thus end up varying the values of the circuit parameters on being introduced. Another example of loading effects is found in the measurement of current by an Ammeter. An ammeter is a very low resistance device that is connected in series in a circuit for the measurement of current. When current passes through it, because of the low finite resistance of the ammeter, there is a small voltage drop across the ammeter which results in power dissipation. This power is again borrowed from the circuit and therefore affects the circuit parameters.

Theoretically loading effects can be reduced to zero by:-

Making the impedance of an instrument that is to be connected in parallel in a circuit as infinite. Making the impedance of an instrument that is to be connected in series in a circuit as zero.

This is because the power dissipation will be zero in an instrument connected in parallel when it will draw no current from the circuit i.e. infinite impedance. Similarly in an instrument connected in series, the power dissipation will be zero when there will be no voltage drop across it i.e. zero impedance.

But practically an infinite or zero impedance in an instrument is not possible. So a condition of minimum loading effects is tried to be reached by making the values of the impedances of instruments very high or very low according to their use. That is why Voltmeters always have very high while Ammeters always have very low impedance values.

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