After discovering the nature of electrical resistance, scientists devised instruments like this one to measure and control it.

In the mid 1820s, Georg Ohm, a German professor of mathematics, began investigating electrical resistance. Through experimentation Ohm determined that all materials exhibit at least some resistance to electric flow, even the most conductive metals. He further found that when the conductive material was the same, a thin wire was more resistant to electricity than a thick one and a long wire was more resistant than a short one. Equipped with this knowledge, scientists were able to develop useful means of varying the resistance in a circuit, such as the rheostat, which was developed in the mid 1800s by  Charles Wheatstone.

Most early rheostats, including the one depicted in this interactive tutorial (a successor to Wheatstone's original), consisted of numerous turns of resistance wire wound around an insulated tube above which a metal bar was positioned. A sliding contact connected the wire to the bar and could be moved to change the amount of coiled wire an electric current traveled through before it was passed on to the bar, which was less resistant to flow.

To operate the tutorial, adjust the rheostat slider. This action moves the contact between the coil and the bar along the length of the tube, changing the resistance of the rheostat. An ammeter is included in the circuit to provide a means of gauging the strength of the current, which varies depending on the resistance of the rheostat. The resistance is lowest when the contact is placed at the far left of the coil, where the current is able to immediately flow through the bar rather than first passing through the much longer and narrower (and thus more electrically resistant) piece of wire composing the coil. Increased resistance occurs as the contact is moved to the right, so that the electricity must travel through an increasing amount of resistance wire coiled around the tube. The more resistance provided by the rheostat, the smaller the current that reaches the ammeter.

Last modified on 17 June 2019
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Magnet Academy is a free resource on magnetism & electricity brought to you by the Center for Integrating Research + Learning at the National High Magnetic Field Laboratory.