You should be able to recognise the graph of current against voltage for a diode. The diode has a very high resistance in one direction. This means that current can only flow in the other direction.

Varistor, By Michael Schmid [CC BY-SA 3.0], via Wikimedia Commons, and voltage-current graph The name varistor doesn’t help much as the name’s origin comes from “varying resistor”, which ...

A direct current flows in only one direction. On a voltage-time graph this would appear as a straight horizontal line at a constant voltage. Car batteries, dry cells and solar cells all provide a ...