Machines such as counters, computers, and Colossus process information. Items of information are given a physical identity which can be recognised and transmitted over distance. The identity is transmitted by what is called a signal and the relationship between the signals and the information they represent is called a code (as distinct from a cipher). Certain types of numbers are commonly used to identify items of information, because they are universally known and endless in quantity: the information, processors, and processing are then said to be digital. Processing consists of logical operations carried out serially on the numbers until a conclusion has been reached, expressed in numbers which can be converted to information by the code. Automatic number processors may be wholly or in part mechanical, electromechanical, or electronic. For the present purpose, electromechanical operation is the easiest to describe and to understand and so will be used as the starting point of this exposition. Electromechanical processors comprise electrical circuits consisting of switches interconnected by wire conductors. A switch is a device which can be set into one of two states, the unoperated or OFF state and the operated or ON state. The earliest switches used movable metal elements called contacts which could be pressed together to connect, or separated to disconnect, two wires that were attached to the contacts. Some metal contact switches used electric motors to operate them. All switches are stable in their OFF states. In their ON states, they may be stable only so long as the operating force is maintained. When this force is removed, they may remain in the ON state or revert to the OFF state. An electric light switch is an example of a manually operated switch which is stable in both the OFF and the ON states. A bell ‘push-button to ring’ switch reverts to the OFF state when the operating force is removed. Electromechanical (i.e. electromagnetically operated) switches usually take the form of relays, each of which consists of a U-shaped iron structure, as shown in Figure 1, around part of which a coil of wire has been wound. A hinged iron armature bridges the open end of the U.
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