current. A complete wave from a to b is called a "cycle" or "period," and the number of cycles occurring in a second is called the "periodicity" or "frequency" of the current. In most electricity works supplying current for lighting the frequency is 50; if the supply is mainly for motive power the frequency is lower, generally 25, and for electric railways a
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still lower standard is likely to be generally accepted.
It will be obvious that, in electric working of main lines of railways, some agreement between the different countries as to a standard frequency is highly desirable, for a change of engine when passing a frontier would be a useless complication in the service. A beginning in the direction of internationally standardising the frequency for electric
