Among the devices that are used to check (continuity) of electrical circuits, not take samples. They are simple in design and application. The probe is an indicator, in series with which is connected, as a rule, galvanic cell (battery), current-limiting resistance and two testers. Touching the probes on different portions of the circuit, checks the condition of all its parts and proper connections.
But in addition to galvanic cells, and other known sources of supply. Why not apply them in the design of the probe and to make the device, for example operating on the basis of electromagnetic induction.
The prospects of this idea are convinced, collecting scheme, much like the school’s demonstration of experience in the study of the laws of Faraday. Source of electricity was multiturn coil (series-connected windings of polarized relay of the RP-4) introduced into the regular “band” magnet 3x3x40, the end of which is connected to a steel spring. As an indicator coming from the led with a nominal current of 20 mA glow. Probes were two cut flexible circuit wire in vinyl insulation, equipped with clips of type “crocodile”.