commercial transmitters, or, in fact, any Radio receiver or transmitter where exceptionally large direct currents are needed.
The elementary D.C. generator is quite like the A.C. generator. The only difference is that the slip rings are replaced by a different kind of current collecting, device, called the “commutator.” Figure 21 shows a drawing of a simple commutator. It is in two parts (segments), each insulated from the other and each is connected to one end of the armature coil, just as the slip rings (or collector rings as they are sometimes called) were in the alternator. Should we connect such a generator to our original voltmeter, and turn the crank of the armature in a clockwise direction, we would find that the needle now reads to the right, returns to zero and then moves to the right again. For every complete revolution of the armature, the needle would move twice from the zero-position.
When the conductor B passes through the north field as in Fig. 21, voltage is generated in B in the direction of the arrow, which is carried through the segment C of the commutator to our voltmeter. Of course, conductor A will supply a voltage in the same direction as before. When conductor B now passes through the South pole in the direction of the arrow, the e.m.f. will be in the opposite direction but note that segment C is now making contact with the other brush, which is the same thing as reversing the terminal of the elementary A.C. generator to the voltmeter. Therefore, as far as the voltmeter is concerned, it is receiving voltage in the same direction. This reversing action takes place twice in one complete rotation of the armature. But our voltage, although acting in the same direction, is not a D.C. current. It is rather a pulsating current and a bad pulsating current at that.
This is exactly the kind of voltage that is generated by a vacuum tube B supply device and we might, if we wanted to, use the same apparatus (a filter) as is used in Radio receivers to smooth out the ripple of P.D.C. Such a device, however, can be used only where small e.m.f.’s are being generated. D.C. generators, you must not forget, are used where there are large demands for D.C. electricity.
Figure 22 shows a simple generator using an electromagnetic field and an armature made up of 8 conductors and using 8 segments. Such a D.C. generator will supply a D.C. voltage--graphically shown in Figure 6.
