The tester must protect itself if you accidentally connect it across an energized circuit or a charged capacitor. Power consumption must be low so that if you accidentally leave the tester on ...

This simple but clever continuity tester sports two LEDs: RED LED = circuit resistance is < approx 10K, GREEN LED = circuit resistance <10Ω. It consumes approx 8mA from a 9V battery. When not ...

It was decided, therefore, to arrange the terminating marker to apply a continuity test for each call to make sure, before it is disconnected, that a circuit is completed to a subscriber line.

Inside, a pager vibration motor is connected, vibrating when continuity is found. The circuit runs from a 1.5V AA battery which neatly fits inside the marker shell.

Even the most basic testers will sense voltage so you can check if a wire is getting power, and continuity so you can test if a circuit is complete, or an earth ground is sound.