How the magnetic effect of current can be used to detect current?

When current flows in a wire, a compass needle kept nearby gets deflected. Even if the current is small, the deflection of the magnetic needle can be seen. We can use the magnetic effect of current to make another tester. For this, take a cardboard tray from inside a discarded matchbox. Wrap an electric wire a few times around the cardboard tray. Place a small compass needle inside it. Now connect one free end of the wire to a terminal of a two cell battery. From the other terminal of the battery, connect another wire. Your tester with two free ends of wire is ready (Fig. 14.7.)