output voltages and currents, and generally do not directly control a final device such as a lamp, solenoid, or motor. Reed switches have small leakage currents compared to solid state devices; this may be useful, for example, in medical devices requiring protection of a patient from tiny leakage currents. The reed is hermetically sealed and can therefore operate in almost any environment, such as where flammable gas is present or where corrosion would affect open switch contacts. A reed switch has very low resistance when closed, typically as low as 50 milliohms, whereas the Hall Effect devices can be in the hundreds of ohms. A reed switch requires only two wires whereas most solid-state devices require three wires. A reed switch can be said to require zero power to operate it. Uses Reed relays One or more reed switches inside an electromagnetic coil constitute a reed relay. Reed relays are used when operating currents are relatively low, and offer high operating speed, good performance with very small currents that are not reliably switched by conventional contacts, high reliability and long life. Millions of reed relays were used in telephone exchanges in the 1970s and 1980s. In particular, they were used for switching in the British TXE family of telephone exchanges. The inert atmosphere around the reed contacts ensures that oxidation will not affect the contact resistance. Mercury-wetted reed relays are sometimes used, especially in high-speed counting circuits. Magnetic sensors Reed switches actuated by magnets are commonly used in mechanical systems as proximity sensors. Examples are door and window sensors in burglar alarm systems and tamperproofing methods. Reed switches have been used in laptops to put the laptop into sleep/hibernation mode when the lid is closed. Speed sensors on bicycle wheels frequently use a reed switch to actuate briefly each time a magnet on the wheel passes the sensor. Reed switches were formerly used in the keyboards for computer terminals, where each key had a magnet and a reed switch actuated by depressing the key. Electric and electronic pedal keyboards used by pipe organ and Hammond organ players often use reed switches, where the glass enclosure of the contacts protects them from dirt, dust, and other particles. They may also be used to control diving equipment, such as flashlights or cameras, which must be sealed to keep water out under high pressure. At one time brushless DC electric motors used reed