A liquid rheostat or water rheostat  or salt water rheostat is a type of variable resistor. 
This may be used as a dummy load or as a starting resistor for large slip ring motors.
In the simplest form it consists of a tank containing brine or other electrolyte solution, in which electrodes are submerged to create an electrical load. The electrodes may be raised or lowered into the liquid to respectively increase or decrease the electrical resistance of the load. To stabilize the load, the mixture must not be allowed to boil.
Modern designs use stainless steel electrodes, and sodium carbonate, or other salts, and do not use the container as one electrode. In some designs the electrodes are fixed and the liquid is raised and lowered by an external cylinder or pump. Motor start systems used for frequent and rapid starts and re-starts, thus a high heat load to the rheostats, may include water circulation to external heat exchangers. In such cases anti-freeze and anti-corrosion additives must be carefully chosen to not change the resistance or support the growth of algae or bacteria.
The salt water rheostat operates at unity power factor and presents a resistance with negligible series inductance compared to a wire wound equivalent, and was widely used by generator assemblers, until 20 years ago, as a matter of course. They are still sometimes constructed on-site for the commissioning of large diesel generators in remote places, where discarded oil drums and scaffold tubes may form an improvised tank and electrodes.