power represented by the A.C. flowing under the potential difference corresponding to the excitation of the magnetic system. Obviously the converse process must also be possible. If we supply electric power in the shape of an A.C. to the armature of this machine and keep the field-magnets excited as before, we must be able to obtain mechanical power from the shaft. But then the speed must be exactly that corresponding to the frequency of the A.C. supplied. To use a machine of this kind as an electric motor we must first bring it up to speed by some means, and only if the speed is exactly such that the rhythm of the passage of the poles in front of the armature coils synchronises with the frequency of the available supply may we switch this on to the armature. Electric motors of this kind are therefore called "synchronous motors." The necessity of bringing the motor first up to speed before being able to switch the driving current on is an inconvenience which renders such motors unsuitable for general purposes.
To the late Professor Ferraris of Turin belongs the merit of having discovered a principle of alternating current working by which the motor may be started by the alternating
