Violet ray devices used a glass electrode to emit a violet-colored electrical discharge when applied to the skin. These devices operated on the principle of high-frequency, low-current electricity. The electricity would pass through a Tesla coil (a transformer designed by Nikola Tesla to produce high-voltage, low-current electricity) before reaching the glass electrodes. The glass tube would glow with a violet light, hence the name, and produce a mild tingling sensation when applied to the body.