Erotic electrostimulation devices should avoid DC currents entirely, in order to avoid electrolytic effects. This is usually achieved through “” waveforms, in which each positive current pulse is followed by an equivalent negative current pulse. Devices with multiple channels (e.g. for several users or body regions) should have a small pulse isolation transformer for galvanic isolation in each channel, such that currents cannot flow across the body between channels. Pulse frequency, duration and amplitude should be selected to achieve the desired stimulation with the least amount of power delivered into the body, for example avoiding current during the refractory period after each action potential, where neurons do not respond to stimuli. Typical erotic electrostimulation devices use pulse frequencies in the range 300–3000 Hz, where skin nerves are most sensitive.