This step consisted in taking the current induced in the revolving helix or armature (by the field magnets) and sending it back through the coils of the field magnets which produced it, thereby increasing the energy of the field magnet coils, and they in turn with an increased efficiency and reciprocal action induce still stronger currents in the armature coils, and so a building up process, or principle of mutual and reciprocal excitation, is carried on until the maximum efficiency is reached. This principle was the discovery of Soren Hjorth, of Copenhagen, and is fully described in his British patent, No. 806 of 1855, for “An Improved Magneto-Electric Battery.” As the prototype of the dynamo, it is worthy of illustration. In the illustration, [Figs. 18] and [19], a is a revolving wheel bearing the armature coils, C permanent magnets, d electro-magnets (field magnets), and g the commutator. Quoting from his specifications, he says: ““The permanent magnets acting on the armatures brought in succession between their poles, induce a current in the coils of the armatures, which current, after having been caused by the commutator to flow in one direction, passes round the electro-magnets (field magnets), charging the same and acting on the armatures. By the mutual action between the electro-magnets and the armatures an accelerating force is obtained, which in result produces electricity greater in quantity and intensity than has heretofore been obtained by similar means.””
Although the principle of the dynamo was clearly embodied in the Hjorth patent, its value was not appreciated until some time later. Eleven years later Wilde (U. S. Pat. No. 59,738, Nov. 13, 1866), employed a small machine with permanent magnets to excite the coil-wound field magnets of a larger machine. But Siemens (British Pat. No. 261 of 1867), taking up the principle employed by Hjorth, dispensed with his superfluous permanent magnets, having found that the residual magnetism, which always remained in iron which has once been magnetized, was sufficient as a basis to start the building up process. Farmer, Wheatstone and Varley also recognized this fact about the same time. Siemens’ patent also was the first embodiment of what is known as the bobbin armature. Gramme and D’Ivernois (British Pat. 1,668 of 1870, and U. S. Pat. No. 120,057, of Oct. 17, 1871), were the first to bring out the continuously wound ring armature.
Active development now began in various types and by various inventors, including Weston, Brush, Edison, Thomson and Houston, Westinghouse, and others, who have brought the dynamo to its present high efficiency.
The revolving coils of the dynamo are called the armature, and the fixed electro-magnets are called the field magnets, and these latter may be two or more in number. When two are used they are arranged on opposite sides of the armature, and form what is known as the bipolar machine. A larger number constitutes the multipolar machine. The field magnets in the multipolar machine usually are arranged in radial position around the entire circumference of the revolving armature, and are held in a fixed circular frame. To give a clear idea of the principles of the dynamo, the bipolar machine is best suited for illustration, and is here given in [Figs. 20] and [21], in which [Fig. 20] represents the dynamo complete, and [Fig. 21] a detail of the end of the armature and commutator. This armature consists of coils or bobbins of insulated wire, each section having its terminals connected with separate insulated plates on the hub, which plates are known as the commutator. When any section of the armature approaches the pole of a field magnet, the current induced in that section of the armature coils by the field magnet, is taken off from a corresponding plate of the commutator by flat springs, seen in [Fig. 20], and known as brushes. The field magnets A and B, [Fig. 20], are shown with only a few turns of wire about them for clearer illustrations of the connections, which are made as follows: The wire a is extended in coils around the field magnet B, and thence around field magnet A, and thence to the upper brush on the commutator, thence through the wire coils or bobbins of the rotary armature C, and thence by the lower brush to the wire b. The terminals of the wires a and b extend to the point of utilization of the current, whether this be electric lights, motors, or other applications. In this illustration, the circuit, it will be seen, passes through both the coils of the field magnets and the coils of the armature, involving the principle of mutual excitation.
FIG. 20.—BIPOLAR DYNAMO.
There are two principal kinds of dynamos—those producing the alternating currents, and those producing the continuous current. In the first the current alternates in direction, or is composed of an infinite number of impulses of opposite polarity: one polarity when a section of the armature coil is approaching a north field magnet pole or receding from a south pole, and the other polarity when receding from a north field magnet pole and approaching a south pole. In the continuous current machine, the commutator and brushes are so arranged as to take up all the impulses of the same polarity and conduct them away by one brush, and gathering all the impulses of the opposite polarity and conducting them away by another brush. Thus the current of each brush, in the continuous current machine, is always of the same polarity, and the polarity of one being always positive, and that of the other negative, the current flows continuously in the same direction. A third species of dynamo is the pulsatory, in which the current flow is invariable in direction, but proceeds in waves.
