<P> Since the high voltages carried in the wires are significantly greater than what is needed in - home, transformers are also used extensively in electronic products to decrease (or step - down) the supply voltage to a level suitable for the low voltage circuits they contain . The transformer also electrically isolates the end user from contact with the supply voltage . Transformers are used to increase (or step - up) voltage before transmitting electrical energy over long distances through wires . Wires have resistance which loses energy through joule heating at a rate corresponding to square of the current . By transforming power to a higher voltage transformers enable economical transmission of power and distribution . Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand . All but a tiny fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer . </P> <P> Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers . Audio transformers allowed telephone circuits to carry on a two - way conversation over a single pair of wires . A balun transformer converts a signal that is referenced to ground to a signal that has balanced voltages to ground, such as between external cables and internal circuits . Transformers made to medical grade standards isolate the users from the direct current . These are found commonly used in conjunction with hospital beds, dentist chairs, and other medical lab equipment . </P> <P> Electromagnetic induction, the principle of the operation of the transformer, was discovered independently by Michael Faraday in 1831, Joseph Henry in 1832, and others . The relationship between EMF and magnetic flux is an equation now known as Faraday's law of induction: </P> <Dl> <Dd> E = d Φ B d t (\ displaystyle (\ mathcal (E)) = \ left ((\ mathrm (d) \ Phi _ (\ text (B))) \ over \ mathrm (d) t) \ right). </Dd> </Dl>

Which of the following is a type of resistance in ac transformer circuit