COMM ufidor or Collector.—These are usually built up of segments of copper or phosphor bronze, insulated from each other as well as from the shaft. The insulation now generally preferred to separate the segments is mica, though in some recent machines of Siemens the collector-bars, made of iron, are separated by air-spaces. The air-space was adopted by a number of constructors in the early stages of electric lighting, among them Weston and Hochhausen, but was discarded on account of the liability to the settling of dust and the bridging of copper particles across the air-space, resulting in the short-circuiting of the arma ture. Connection between commutator and armature wires should in all cases he soldered, as screws are apt to work loose. The commutator requires constant care, and should not be allowed to wear into grooves or ruts, which eventually give rise to destructive sparking. For lubrication, oil is avoided if possible. as it is apt to settle among the bars, harden, and carbon ize, and finally short-circuit the bars. For that reason French chalk is frequently employed; but more recently the application of carbon brushes has overcome many diffi culties connected with the commutator. Another class of commutator, some times employed for self-exciting, alternate-current machines, is shown in Fig. 15.
are employed to take the current from the commutator bars and deliver it to the working-circuit. Various forms are employed, among them those shown in Fig. 16. The object in all cases is to secure as good a contact as possibte between commutator and brush, and hence the lat ter has been given the forms shown. In A a number of copper wires are grouped into a brush soldered together at their ends. In B a flat strip is slit longitudi nally, while in U a series of strips are soldered together and bear c Igewise on the commutator. Within the past few years "carbon brushes," as they are called, have come into extensive use, especially in connection with motors. They are mode up of a pressed mass containing coke and a certain percentage of plumbago, which gives them excellent lubri cating qualities. Their great merit, however, lies in the fact that they do not burn perceptibly, and hence have a long life, at the same time protecting the commutator from wear. Brushes made of copper wire-gauze are also largely in use.
Method of connecting Armature and methods of the connection of the armature to the tield-magnet, as well as the [node of excitation of the dynamo-machine, are most intimately connected with its regu lating properties. Magnetism may be excited in the field magnets in various ways.
1. type, shown in Fig. 17, is the oldest employed, and has permanent steel magnets. This form is still used in small machines for special purposes, as in magneto-calls, telephones, etc., and for experimental work, but has long been discarded in large machines on account of the great weight of the machine for a given output. and also because the permanent magnets gradually diminish in strength,
and thus reduce the output of the machine. On account of their simplicity, however, perma nent magnets are still employed in machines of the De Meritens type. intended for light house work. (See Alliance. Pi.r i, and other magneto-machines, pp. 519, 520, old edition.) 2. Separately Excited type was employed by Faraday and later by Wilde in 1866 (see p. 522, old edition). This machine, as well as the magneto-dynamo, has the field magnetism constant, and hence the E. M. F. generated is independent of changes in the ex ternal circuit, Both the preceding types of machines may lie regulated either. by altering the speed or by varying the magnetism passing through the armature. Fig. 18 shows the method of connection.
i. .S'erips-Dynaneo.—This is the type of machine now generally employed for arc-lighting, and which is speeially mlapted to furnish currents of constant strength. As shown in Fig. 19, the armature, the external circuit, and the field-magnet. windings, are all connected in series, so t hat the eurrent is of equal sttengih in all parts of the circuit. This type of ma chine does not begin to generate current until it has attained a certain "critical " speed, as below this speed the, magnets do not become excited ; this speed depends upon the resistance t he droll). This type of machine is also liable to have its polarity reversed ; hence, it is not employed in cleetro-plating or charging storage-hat h-ries, (1. Shaut-Dynarno.—This type is the One most generally employed at the present time for conmant potential in:whines, such as are used for incandescent-lighting, The connections are shown in Fig. 20. The armature here feeds two independent eircuits: (a) the main circuit, which commas with t he lamps. and is indicated by t he heavy line. and arrow ; (h) the shunt circuit, which energizes the field-magnets. The sl unt circuit, consists of fine wire, usually measuring several hundred times the resistance of the armature, and is so arranged that it takes only a small fraction of the total current of the machine (usually not exceeding from 3 to 5 per cent). According to theory, a machine of this type, having no resistance in the arma tare and an infinite resistance in the shunt circuit, ought to be selt-regulating—that is, when kept at constant speed, the potential, or E. 31. F., remains constant, no mutter what the load on the external circuit. In practice these conditions are, of course, impossible to carry out. But machines are now frequently built in which the ratio of armature resistance to shunt re sistance is so great that the regulation is practically perfect.