must not be accidentally switched off, for example, corridor and restroom lighting controls in public buildings such as schools, a tamper-resistant switch may be installed. These require a key to operate and so discourage casual or accidental operation of the switch. Voltage class In North American commercial and industrial lighting installations, lighting installed on 480Y/277 V 3-phase circuits uses voltages higher than the rating of common 120 V switches. Mercury switch Around the middle of the 20th century, "quiet" switches were introduced that used mercury to make the connection, avoiding the loud snap made by the spring-loaded toggle switches that were then the norm. One form of mercury light switch uses a sealed glass vial that is tipped by the toggle lever, causing a drop of liquid metallic mercury to roll from one end to the other, bridging a pair of contacts to complete the circuit. Another style of mercury light switch uses a metal wheel with an insulator separating the two halves that has a hole which allows the mercury to connect either side of the wheel in only one position. Some of these switches were also equipped with a neon lamp connected across the contacts, and thus in series with the electrical load. This caused the indicator to glow faintly when the switch was off, as an aid to finding the switch in a dark room. By the 1970s, other quiet switch designs that did not require mercury (and were thus simpler and cheaper to produce) came into widespread use, gradually replacing both conventional "snap" toggle switches and mercury-based quiet switches. Pull-chain or pull-cord A light switch combined with a light socket is sometimes installed in basement or utility areas of homes. The switch is operated by a pull chain or cord. It is also possible to have the cord-operated switch separate from the light socket, which is particularly common in British bathrooms. Until 2001, UK wiring regulations required that all bathroom switches were operated by pull cords. Dimmer switch A dimmer switch, properly called a dimmer control, contains a solid-state circuit to allow changing the brightness by reducing the average voltage applied to the lamp. Electronic switches In principle, it is easy to design silent switches in which the mechanical contacts do not directly control the current, but simply signal a solid-state device such as a thyristor to complete the circuit. Many variations on this theme have