Figs. 2,585 and 2,586.—Oscillograms (from paper by Morris and Catterson-Smith, Proc. I. E. E., Vol. XXXIII, page 1,023), showing how the current varies in one of the armature coils of a direct current motor. Fig. 2,585 was obtained with the brushes in the neutral position, and fig. 2,586 with the brushes shifted forward.
The various methods of determining the wave form may be further classified as:
| ❴ Joubert's method; | |
| ❴ Four part commutator method; | |
| ❴ Modified four part commutator method; | |
| 1. Step by step | ❴ Ballistic galvanometer method; |
| ❴ Zero method; | |
| ❴ By Hospitalier ondograph. |
Fig. 2,587.—Oscillogram by Bailey and Cleghorne (Proc. I.E.E., Vol. XXXVIII), showing the sparking pressure or pressure between the brush and the commutator segment at the moment of separation. The waves fall into groups of three owing to the fact that there were three armature coils in each slot.
| ❴ cathode ray; | ||
| ❴ by use of various types | ❴ glow light; | |
| 2. constantly recording | ❴ of oscillograph, | ❴ moving iron; |
| ❴ such as | ❴ moving coil; | |
| ❴ hot wire. | ||
Fig. 2,588.—Various wave forms. The sine wave represents a current or pressure which varies according to the sine law. A distorted wave is due to the properties of the circuit, for instance, the effect of hysteresis in an iron core introduced into a coil is to distort the current wave by adding harmonics so that the ascending and descending portions may not be symmetrical. A peaked wave has a large maximum as compared with its virtual value. A peaked wave is produced by a machine with concentrated winding.