and David Geringer of Security Door Controls, an access control hardware manufacturing firm. The device outlined in their designs was the same in principle as the modern magnetic lock consisting of an electromagnet and an armature plate. The patent did not make any reference to the manufacturing methods of the electromagnet and detailed several variations on the design, including one that used a spring-loaded armature plate to bring the armature plate closer to the electromagnet. The patent expired on May 2, 2009. Holding force A magnetic lock has a metal plate surrounded by a coil of wire that can be magnetized. The number of coils determines the holding force which characterizes the lock: Micro Size: holding force. Mini Size: holding force Midi Size: holding force Standard Size: holding force. Shear Lock: holding force The standard size electromagnetic lock is used as a gate lock. Electrical requirements The power for an electromagnet lock is DC (direct current), around 5–6 W. The current is around 0.5 A when the voltage supply is 12 VDC and 0.25 A when using 24 VDC (varies between manufacturers and if there are one or two coils in the block). It is also recommended to verify that the magnetic lock carries the UL mark. Generally, the specification of the electromagnet lock is dual voltage of 12/24 VDC.. If using a rectifier to convert AC power, a full wave bridge rectifier should be used. Application modes For safety purposes, electric locks and electric strikes can be designed to operate in one of two modes: Fail-safe – to protect people: The lock is released if power cuts off. Fail-secure (also known as non fail-safe) – to protect property: The lock remains closed if power cuts off. This type of lock has a cylinder, similar to those found in conventional locks. The cylinder makes it possible for the lock to remain secure, even if the power supply is cut off. Electromagnetic locks are usually fail-safe and must satisfy applicable fire regulations so as to be safe in emergency situations. References Locks (security device) American inventions