Variable resistors, such as the potentiometer and the rheostat, have ohmic characteristics. The term "rheostat" is becoming obsolete, with the general term "potentiometer" replacing it. One adjusts the rheostat control to increase or decrease the voltage. Fan speed is typically controlled via a step-wise or rheostat motor adjustment. The filament rheostat was also used as a volume control. The light's brilliance was adjustable by rheostat, but the control was ineffective. Aurora had a plunger design in which a stack of carbon/silicon discs replaced the rheostat. The motor is controlled by a rheostat (think sewing machine foot pedal). Otherwise the power is dissipated through a braking rheostat. Most early examples had vertical, thumb-operated plungers with the rheostat in the grip. Agatha peruses a manual for the Bergonic chair and begins to experiment with the rheostat. Such a device is called a rheostat and has two terminals. Also an inventor, she patented nine inventions related to her instrument, including the rheostat. Rarely a salt water rheostat is used. It consists of a parallel and a series rheostat connected in an "L" configuration.