How a rheostat works
Including a longer and longer wire in series
A rheostat is a very simple variable resistor. It's made by winding a very thinly insulated resistance wire around a barrel. A metal slider or 'jockey' wears away a line of the insulation so it can make electrical contact with the metal underneath.
The jockey is mounted on a thick metal bar and this makes up part of the circuit. As the jockey is moved along the metal bar it includes more and more of the resistance wire in the circuit.
The longer the wire in series with a bulb the higher the resistance and the dimmer the bulb.
A rheostat actually has three terminals so can be connected as a simple variable resistor in series or as a potential divider.