[Illustration:  Fig. 72.  Generator with Magnets Removed]

As is to be expected from any two-pole alternating generator, there is one cycle of current for each revolution of the armature.  Under ordinary conditions a person is able to turn the generator handle at the rate of about two hundred revolutions a minute, and as the ratio of gearing is about five to one, this results in about one thousand revolutions per minute of the generator, and, therefore, in a current of about one thousand cycles per minute, this varying widely according to the person who is doing the turning.

[Illustration:  HOWARD OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO An All-Concrete Building Serving the District South of Market Street.]

The end plates which support the bearings for the armature are usually extended upwardly, as shown in Fig. 72, so as to afford bearings for the crank shaft.  The crank shaft carries a large spur gear which meshes with a pinion in the end of the armature shaft, so that the user may cause the armature to revolve rapidly.  The construction shown in Fig. 72 is typical of that of a modern magneto generator, it being understood that the permanent magnets are removed for clearness of illustration.

Fig. 73 is a view of a completely assembled generator such as is used for service requiring a comparatively heavy output.  Other types of generators having two, three, or four permanent magnets instead of five, as shown in this figure, are also standard.

[Illustration:  Fig. 73.  Five-Bar Generator]

Referring again to Fig. 69, it will be remembered that one end of the armature winding shown diagrammatically in that figure, is terminated in the pin 5, while the other terminates in the pin 7.  When the armature is assembled in the frame of the generator it is evident that the frame itself is in metallic connection with one end of the armature winding, since the pin 5 is in metallic contact with the armature casting and this is in contact with the frame of the generator through the bearings.  The frame of the machine is, therefore, one terminal of the generator.  When the generator is assembled a spring of one form or another always rests against the terminal pin 7 of the armature so as to form a terminal for the armature winding of such a nature as to permit the armature to rotate freely.  Such spring, therefore, forms the other terminal of the generator.

Automatic Shunt.  Under nearly all conditions of practice it is desirable to have the generator automatically perform some switching function when it is operated.  As an example, when the generator is connected so that its armature is in series in a telephone line, it is quite obvious that the presence of the resistance and the impedance of the armature winding would be objectionable if left in the circuit through which the voice currents had to pass.  For this reason, what is termed an automatic shunt is employed on generators designed for series work; this shunt is so arranged that it will automatically shunt or short-circuit the armature winding when it is at rest and also break this shunt when the generator is operated, so as to allow the current to pass to line.