Direct current (DC) is electricity flowing in a constant direction. So, electrons always flow constantly in the same direction within the electrical circuit, and/or possessing a voltage with constant polarity, therefore they will always circulate in the same direction.
Direct current was produced in 1800 by Italian physicist Alessandro Volta’s battery, by his “Voltaic pile.” Direct current is the type of electricity made by a battery (with definite positive and negative terminals), or the kind of charge generated by rubbing certain types of materials against each other. Direct current was adopted by Thomas Edison in the late 19th century at the beginning of industrial electrical distribution. Subsequently, however, the technology moved to alternating current, invented by Nikola Tesla, more convenient for the transmission of electricity remotely. The efficiency of the alternating current made it possible to drastically decrease energy losses over long distances, thanks to the increase in electrical voltage which allowed to transmit high electrical power at high voltage and low current, drastically reducing losses due to dissipation on the line and therefore the thickness of the electrical conductor used for transport, compared to Edison direct current.
In a direct current system, unlike in alternating current ones, it is very important to respect the direction of the current, that is the polarity. There are in fact in the batteries a “positive” and a “negative” pole, which must be correctly connected to the load. For example, a DC motor, if powered backward, rotates in the opposite direction, unlike a single-phase AC motor. Many electronic circuits, if powered incorrectly, can fail, particularly if they are not protected by an anti-reverse diode.
The direct current can be produced both with a dynamo and through an alternator starting from an alternating current (AC) with, following, a rectification process (by use of a rectifier), carried out with diodes or rectifiers bridges.