A digital multimeter makes a good frequency meter with the addition of the circuit shown.

Sometimes, a more complex model is required such as the equivalent circuit shown in figure 4.

Then the two circuits shown below are equivalent:

For instance, the three circuits shown in figure 1.1 all look different but have identical topologies.

Note that with the circuit as shown, if V changes, the output current will change.

Output resistance is found using a small-signal model for the circuit, shown in Figure 2.

The circuit shown computes the difference of two voltages multiplied by some constant.

An external 2-pole 5-way switch will then be needed and the circuit shown in Fig. 13 built up on a small board.

With the inductor in the circuit as shown, the behavior is completely different.

The system we consider is the electrical circuit shown in Fig. 24.5.