If the system of primary coils is so arranged as to give the impression that one pair of poles, that is, a 2-pole pattern (one north pole and one south pole) are contained in the full 360°, the field will rotate one revolution in one complete cycle of the AC mains supply. In Europe the supply frequency is usually 50 cycles per second (denoted '50 hertz'). In America the standard frequency is 60 hertz. Thus a 2-pole motor On 50 Hz supply has a field speed of 50 revolutions per second or 3000 revolutions per minute (rpm). If the primary coils are arranged to give a 4-pole pattern, the speed is halved, and so on, so the possible synchronous motor speeds in Europe are 3000, 1500, 1000, 750, 600, 500 and so on, rpm. Corresponding induction motor speeds are less than these values by the small amount of slip necessary to induce secondary current. A disadvantage of the synchronous motor is that it cannot self-start and the speed must he brought to within a very small percentage of synchronism before magnetic 'locking' can take place. The synchronous machine is more commonly used as a generator or ALTERNATOR, almost all the power stations in Britain employing machines of this type