<P> Direct current was produced in 1800 in Italian Physicist Alessandro Volta's battery, his Voltaic pile . The nature of how current flowed was not understood . French physicist André - Marie Ampère conjectured that current travelled in one direction from positive to negative . When French instrument maker Hippolyte Pixii built the first dynamo electric generator in 1832, he found that as the magnet used passed the loops of wire each half turn, it caused the flow of electricity to reverse, generating an alternating current . At Ampère's suggestion, Pixii later added a commutator, a type of "switch" where contacts on the shaft work with "brush" contacts to produce direct current . </P> <P> The late 1870s and early 1880s saw electricity starting to be generated at power stations . These were initially set up to power arc lighting (a popular type of street lighting) running on very high voltage (usually higher than 3000 volt) direct current or alternating current . This was followed by the wide spread use of low voltage direct current for indoor electric lighting in business and homes after inventor Thomas Edison launched his incandescent bulb based electric "utility" in 1882 . Because of the significant advantages of alternating current over direct current in using transformers to raise and lower voltages to allow much longer transmission distances, direct current was replaced over the next few decades by alternating current in power delivery . In the mid-1950s, high - voltage direct current transmission was developed, and is now an option instead of long - distance high voltage alternating current systems . For long distance underseas cables (e.g. between countries, such as NorNed), this DC option is the only technically feasible option . For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current . </P> <P> The term DC is used to refer to power systems that use only one polarity of voltage or current, and to refer to the constant, zero - frequency, or slowly varying local mean value of a voltage or current . For example, the voltage across a DC voltage source is constant as is the current through a DC current source . The DC solution of an electric circuit is the solution where all voltages and currents are constant . It can be shown that any stationary voltage or current waveform can be decomposed into a sum of a DC component and a zero - mean time - varying component; the DC component is defined to be the expected value, or the average value of the voltage or current over all time . </P> <P> Although DC stands for "direct current", DC often refers to "constant polarity". Under this definition, DC voltages can vary in time, as seen in the raw output of a rectifier or the fluctuating voice signal on a telephone line . </P>

What is the frequency of a dc voltage