magnet. A polarity controlled relay needs changeover switches or an H-bridge drive circuit to control it. The relay may be less expensive than other types, but this is partly offset by the increased costs in the external circuit. In another type, a ratchet relay has a ratchet mechanism that holds the contacts closed after the coil is momentarily energized. A second impulse, in the same or a separate coil, releases the contacts. This type may be found in certain cars, for headlamp dipping and other functions where alternating operation on each switch actuation is needed. A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges. An earth-leakage circuit breaker includes a specialized latching relay. Very early computers often stored bits in a magnetically latching relay, such as ferreed or the later remreed in the 1ESS switch. Some early computers used ordinary relays as a kind of latch—they store bits in ordinary wire-spring relays or reed relays by feeding an output wire back as an input, resulting in a feedback loop or sequential circuit. Such an electrically latching relay requires continuous power to maintain state, unlike magnetically latching relays or mechanically ratcheting relays. In computer memories, latching relays and other relays were replaced by delay-line memory, which in turn was replaced by a series of ever faster and ever smaller memory technologies. Machine tool relay A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally open to normally closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel. Although such relays once were the backbone of automation in such industries as automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications. A relay allows circuits to be switched by electrical equipment: for example, a timer circuit with a relay could switch power at a preset time. For many years relays were the standard method of controlling industrial electronic systems. A number of relays could be used together to carry out complex functions (relay logic). The principle of relay logic is based on relays which energize and de-energize associated contacts. Relay logic is the predecessor of ladder logic, which