Devices for measuring currents and voltages in a circuit are referred to as ammeters and voltmeters, respectively. In each case, the ideal situation is for the meter to measure the desired quantity without altering the characteristics of the circuit being studied. This is accomplished in different ways for these two types of meters.

Figure 26 (a) shows the connection of an ammeter in a circuit. Since an ammeter is an instrument that measures current, it must be inserted in the circuit so the current passes directly through it. When an ammeter is inserted into a circuit, the internal resistance of the ammeter adds to the circuit resistance. Any increase in circuit resistance causes a reduction in current, and this is a problem, because an ammeter should only measure the current, not change it. Therefore, an ideal ammeter would have zero internal resistance.

A voltmeter is connected “externally,” in parallel with the circuit across which the voltage is to be measured. It measures the potential difference between two points. Its two wire leads are connected to the two points a and b as shown in Figure 26 (b), where the voltage across R1 is being measured.

A real voltmeter always allows some current to flow through it, which means that the current flowing through the circuit is less than before the meter was connected. As a result, the measured voltage is altered from its ideal value.