Which type of equipment requires two electrodes to function?

Electrotherapy modalities require two electrodes to function because they work by passing a controlled electrical current through the body to stimulate nerves and muscles. The two electrodes are essential for creating a closed circuit; one electrode serves as a positive (anode) and the other as a negative (cathode), allowing for the flow of current between them. This movement of electrical energy can help alleviate pain, improve circulation, and promote healing within the tissues being treated.

In contrast, while heating devices may also utilize electrical current, they typically do not require two distinct electrodes for operations and instead may function with a single power source. Insulation testers and batteries also operate on principles that don't necessitate two electrodes in the way that electrotherapy does. Thus, the unique requirement for dual electrodes in electrotherapy modalities distinguishes it as the correct answer.