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

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

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

Loading Effect Formula:

\text{Loading Effect} = \left(\frac{{\text{Theoretical value} - \text{Measured value}}}{{\text{Theoretical value}}}\right) \times 100

Loading Effect in Electrical Measurements

Loading effect of voltmeters

A voltmeter always connects in parallel to the electrical components for measuring the voltage. Practical voltmeters are designed to possess very high internal resistance (usually 10 or 20 MΩ). In case of voltmeters there are two cases that might result in loading effect leading to faulty measurements.

  1. When internal resistance of voltmeter is small and it connects to external resistance. The overall resistance will be changed.
  2. 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.

Case 1 Illustration: A voltmeter with small internal resistance

Consider a voltmeter having internal resistance of 100 ohms connected in a series circuit having 12 V source and 100 ohms resistor.

The internal resistance of voltmeter will dissipate some voltage. The terminal voltage across voltmeter can be calculated using Kirchhoff’s Voltage law as shown below:

Loading Effect = (12 V – 6 V)/12 V * 100% = 50%

From above, one can conclude that a voltmeter having small internal resistance will dissipate voltage across it. In present case it is 50% which is quite high and practically not an acceptable value. The previous case is a theoretical illustration only, since the practical voltmeters are designed with internal resistances of order of few mega ohms. Let’s consider a practical case below.

Case II: A voltmeter measuring voltage across very high resistance

Consider a voltmeter having 10 MΩ internal resistance connected to a series circuit having 12 V source and 1 Mohms resistor.

The application of Kirchhoff’s voltage law enables to calculated the loading effect of voltmeter as follows:

Loading Effect = (12 V – 10.9 V)/12 V * 100% = 9.16%

From above case one can conclude that if the resistance of measuring device (here digital voltmeter) is quite large as compared to the circuit resistance, the meter will yield quite appropriate results. In practice if resistance of voltmeter is 10 times the resistance, the results are acceptable.

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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