Such an arrangement is indicated diagrammatically in Fig. 77.  Instead of having one terminal of the armature winding brought out through the frame of the generator as is ordinarily done, both terminals are brought out to a commuting device carried on the end of the armature shaft.  Thus, one end of the loop representing the armature winding is shown connected directly to the armature pin 1, against which bears a spring 2, in the usual manner.  The other end of the armature winding is carried directly to a disk 3, mounted on but insulated from the shaft and revolving therewith.  One-half of the circumferential surface of this disk is of insulating material 4 and a spring 5 rests against this disk and bears alternately upon the conducting portion 3 or the insulating portion 4, according to the position of the armature in its revolution.  It is obvious that when the generator armature is in the position shown the circuit through it is from the spring 2 to the pin 1; thence to one terminal of the armature loop; thence through the loop and back to the disk 3 and out by the spring 5.  If, however, the armature were turned slightly, the spring 5 would rest on the insulating portion 4 and the circuit would be broken.

[Illustration:  Fig. 77.  Pulsating-Current Commutator]

[Illustration:  Fig. 78.  Generator Symbols]

It is obvious that if the brush 5 is so disposed as to make contact with the disk 3 only during that portion of the revolution while positive current is being generated, the generator will produce positive pulsations of current, all the negative ones being cut out.  If, on the other hand, the spring 5 may be made to bear on the opposite side of the disk, then it is evident that the positive impulses would all be cut out and the generator would develop only negative impulses.  Such a generator is termed a “direct-current” generator or a “pulsating-current” generator.

The symbols for magneto or hand generators usually embody a simplified side view, showing the crank and the gears on one side and the shunting or other switching device on the other.  Thus in Fig. 78 are shown three such symbols, differing from each other only in the details of the switching device.  The one at the left shows the simple shunt, adapted to short-circuit the generator at all times save when it is in operation.  The one in the center shows the cut-in, of which another form is described in connection with Fig. 75; while the symbol at the right of Fig. 78 is of the make-and-break device, discussed in connection with Fig. 76.  In such diagrammatic representations of generators it is usual to somewhat exaggerate the size of the switching springs, in order to make clear their action in respect to the circuit connections in which the generator is used.