Loading Effects of Instruments

In the previous section (Internal resistance of voltage sources), we learned that power supplies are not the ideal instruments we may have thought they were. The applied load can have an effect on the terminal voltage. Fortunately, since today's supplies have such small load regulation factors, the change in terminal voltage with load can usually be ignored for most applications. If we now turn our attention to the various meters we use in the lab, we again find that they are not totally ideal:
Whenever you apply a meter to a circuit, you change the circuit and the response of the system. Fortunately, however, for most applications, considering the meters to be ideal is a valid approximation as long as certain factors are considered.
For instance,
Any ammeter connected in a series circuit will introduce resistance to the series combination that will affect the current and voltages of the configuration.
Fig. 1: Including the effects of the internal resistance of an ammeter: (a) 2 mA scale; (b) 2 A scale.
The resistance between the terminals of an ammeter is determined by the chosen scale of the ammeter. In general,
for ammeters, the higher the maximum value of the current for a particular scale, the smaller will the internal resistance be.
In Fig. 1(a), the ammeter scale is $2mA$, which means the maximum current flow through ammeter should not exceed $2mA.$ while Fig. 1(b), shows the ammeter with scale $2A$, which is far higher than $2mA$ scale. The $2mA$ scale ammeter series resistance must be high e.g $250Ω$ enough to resist the current flow in the meter circuit to limit the value upto $2mA$ current . While the $2A$ scale ammeter series resistor must be small enough e.g $1.5Ω$ to prevent current flow in the meter circuit up to $2A$ current. In the ideal situation of ammeter connection to the circuit, we ignore the internal resistance of the circuit inside ammeter instrument due to low internal series resistance.

How does ammeter effect circuit analysis?

In the Fig. 2(a), the ideal consideration of ammeter does not have any effect on the current flow in the series circuit. while in Fig. 2(b), the internal resistance $250Ω$ is summed with the load resistors and the result is shown as:
Fig. 2: Applying ammeter to a circuit with resistors in KΩ: (a) Ideal. (b) Practical.
For example, in Fig. 2(a), for an ideal ammeter, the current displayed is $0.6 mA$ as determined from
$$I_s = E / R_T = 12 V/20 kΩ = 0.6 mA$$
If we now insert a meter with an internal resistance of $250 Ω$ as shown in Fig. 2(b), the additional resistance in the circuit will drop the current to $0.593 mA$ as determined from
$$I_s = E/R_T = 12 V/20.25 kΩ = 0.593 mA$$
Now, certainly the current has dropped from the ideal level, but the difference in results is only about 1% - nothing major, and the measurement can be used for most purposes. If the series resistors were in the same range as the $250 Ω$ resistors, we would have a different problem, and we would have to look at the results very carefully.

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