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Electrical current is the flow of positively charged particles from a pole with a higher voltage to a pole with a lower voltage (Loeb & Gans, 1986). The pole termed the anode (designated as +) is the current source; the other pole (cathode) attracts positively charged particles (designated as -). Because of accumulation of positive charges at the cathode, the current sink, neural stimulation occurs at the cathode (Dumitru, Amato, & Zwarts, 2002).
Electrical current flows more easily through media with the least resistance, such as salt solutions, in contrast with air, which has much higher resistance. Current flow can be continuous and direct (DC) or alternating (AC), in which the direction of current alternates between positive and negative directions as the polarity of the two electrodes alternates. Most clinical applications provide pulses of electrical current that can be either biphasic (alternating between the two directions) or monophasic (the current always flows only in one direction).
Pulse width is the duration of a single pulse (usually 100-500 microseconds, µs); frequency is the rate at which pulses are applied (usually 10-60 pulses per second, pps or Hertz, Hz). Current amplitude is the rate of flow of charged particles per second and is measured in amperes (1 coulombe of charge per second=1 A). Voltage is the force used to move current between two poles, referred to as the potential difference between two poles. The amount of current flow between two poles having a particular potential difference will depend upon the tissue and electrode impedance, which is resistance to current flow, and electrode orientation. Therefore, knowing a voltage difference does not indicate what current will flow because of differences in tissue and electrode characteristics. It is safer to use a stimulation device that measures the amount of current passing between the two poles rather than the voltage being applied.
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