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# Wheatstone Bridge – 1843

This device for measuring resistance in a circuit, still widely used today, was "discovered" in 1843, but had been invented a decade earlier. The inventor's name was not Wheatstone.

Ever heard of the Christie bridge? We didn’t think so. Yet that’s what a famous electrical circuit, first devised in the 19th century, would have been called had it been named after its inventor. Instead the apparatus, composed of four resistors, a battery and a galvanometer, was named for the man who popularized it — Sir Charles Wheatstone.

A scientist and mathematician, Samuel Hunter Christie, developed the circuit to measure unknown electrical resistances and first described it in 1833. The bridge worked because of the special diamond-shaped arrangement of the four resistors. Electrical current from a battery split into two parallel branches of the circuit. One consisted of a resistor with a fixed, known resistance and an adjustable resistor, also with a known resistance. The other leg contained a resistor of fixed and known resistance and another whose resistance needed to be determined. By using a galvanometer to balance the current flowing through the two branches, Christie could, with the help of a little math, determine the value of the unknown resistor.

For a decade few people knew of the bridge. Then another British scientist, Wheatstone, came across Christie’s description of the instrument, which Wheatstone referred to as a “differential resistance measurer.” A prominent member of the Royal Society of London, Wheatstone was well-positioned to give the tool a popularity boost. He gave an account of Christie’s invention at an 1843 lecture, and soon after it came to be called the Wheatstone bridge and was used in telegraphy and other applications. Wheatstone himself, however, gave full credit for its invention to Christie. But in translations of his lecture that appeared in Germany and France the following year, Wheatstone’s attribution was nowhere to be found.

In addition to bringing the device to public attention, Wheatstone improved the design (Wheatstone developed the rheostat, a variable resistor) and found several new uses for it. By changing the type of elements contained in its legs, the Wheatstone bridge can determine unknown capacitances, inductances, frequencies and other properties. Besides Wheatstone, several other scientists helped extend the range of the device, including William Thomson, Lord Kelvin and James Clerk Maxwell. This sensitive, accurate method for measuring resistance is still widely used today.