Varistor, By Michael Schmid [CC BY-SA 3.0], via Wikimedia Commons, and voltage-current graph The name varistor ... Each of these lightning arresters contains a Metal Oxide Varistor (MOV).

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.

The current through them is not directly proportional to the potential difference across them. Thermistors close thermistorAn electrical device whose resistance decreases as its temperature increases.