# The History of the Rheostat

A rheostat is a device with a resistance to electric current is dependent on the position of some type of mechanical element or control within the device. The rheostat offers an interesting history.

## What's in a Name?

Wheatstone makes full use of the name rheostat. "Rheo" is of Greek origin and means "to flow." A rheometer can be described as a source of electric current, whereas a rheoscope is a device-to-device used to measure current. A rheostat is a device used to maintain a constant current. Wheatstone invented the rheostat in 1843.

## The Wheatstone Bridge

Wheatstone proposed that a rheostat be placed in a series with a rheoscope and a rheomotor. The reading of the rheoscope was then noted and a rheostat was inserted in the circuit in place of what was the unknown. The rheostat was then adjusted to give the same current reading. The Wheatstone Bridge was used to calibrate the rheostat.

The Wheatstone Bridge was not invented by Charles Wheatstone, but rather by Hunter Christie. Wheatstone, however, was responsible for popularizing the arrangement of four resistors, giving Christie full credit in his 1843 Bakerian Lecture.

## Physics Departments

At one time, physics departments were heavily equipped with tubular rheostats. They were used to pick off a certain fraction of an EMF.

The Bakerian Lecture is the Royal Society's "premier" lecture in the physical sciences. Originating in 1775, the lecture is still ongoing today. The 2010 Bakerian Lecture will be given by Professor Donal Bradley, FRS in March of 2010.

## How Does a Rheostat Work?

The basic principle of a rheostat is Ohm's law. Ohm's law states that "current is inversely proportional to resistance for a given voltage." That pretty much means that the current will decrease as the resistance increases. It also means that it may increase as the resistance decreases. The current enters the rheostat through one of the terminals. It flows through the wire coil and contact, exiting through the terminal.