What is Loading Effect of Electrical Measurement Instruments [Multimeters, Voltmeters, Ammeters]

Question: What is the loading effect of Electrical measurement instruments?

Answer: Loading effect is the degree to which a measurement instrument impacts electrical properties (voltage, current, resistance) of a circuit.


Explanation

Loading effect of voltmeters

A voltmeter always connects in parallel to electronic components for measuring the voltage. Practical voltmeters are designed to possess very high internal resistance (usually 10 or 20 MΩ). There are two cases which might result in fault measurements.

  1. When voltmeter connects to a very high external resistance the overall resistance of circuit changes. For example, a voltmeter with 10 MΩ internal resistance while connected in parallel to 7.5 MΩ resistors will change overall resistance to 4.28 MΩ.
  2. When internal resistance of voltmeter is small and it connects to external resistance. The overall resistance will be changed.

Both above cases change the working of the circuit and thus lead to inappropriate readings.

Loading effect of the ammeter

The ammeter connects in series for measuring current flow through a circuit. A practical ammeter is built with very small internal resistance (10 – 100 Ω). Two possible cases which impact working of ammeter are:

  1. When ammeter connects to a low resistance circuit the overall resistance of circuit stretches to a noticeable amount. e.g when a meter with 50 ohms internal resistance connects to a 100 ohms circuit the overall resistance of circuit increases by 3 times. This also impacts current and voltage by 3 times.
  2. When internal resistance of meter is high the same impact results and electrical parameters of circuit vary.

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