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. A few cases of accidental death as a result of autoerotic electrostimulation have been reported in the forensic science literature; the cases reported involved mains-powered, self-made devices, with current passing through the chest intentionally (usually via nipple stimulation) or unintentionally (for instance, touching an energized part with a hand). In one case reported in the press, a man from York, Pennsylvania was sentenced to 20–40 years in prison for third-degree murder and reckless endangerment after killing his wife with electrostimulation to her nipples directly from a power strip plugged into the mains. History The use of electricity for entertainment purposes dates back at least as early as the 1740s. In the 1830s, insertable electrode attachments for small magnetos could be purchased. Later in the 1800s, various electric belts (some complete with "suspensory sack") were advertised as cures for impotence. In the 1920s, the American Medical Association investigated such devices, and concluded that they provided "more or less mechanical masturbation". Modern electrostimulation first became recognized during the 1950s with the introduction of a device called the Relax-A-Cizor, which was originally designed to stimulate the muscles of a relaxing subject using electric currents as a means of "passive exercise". Such power sources are still in medical use today and are known as EMS (electrical muscle stimulation) units. Some people soon found alternative uses for the Relax-A-Cizors by placing the contacts on sexual parts of the body. By the 1970s, medical TENS (transcutaneous electrical nerve stimulation) units were also being used for electrostimulation. In the 1980s the first devices manufactured specifically for erotic electrostimulation became available, in particular the Titillator and the Pleasure Box, later known as the PES Power Box. In the 1970s, experimenters noticed that bare speaker wires could deliver a jolt and began using recorded and live sound for electrostimulation. At that time, there were no professionally made attachments for such play, so people built their own out of copper plumbing parts and other metal