:3. Yule-Armature$.—In this type the g,enerating, coils are wound on iron cores projecting radially from the axis. This type has been employed by Lontin, Gr•avuure, and others, and by Weston in his electroplating machines. It is now practically obsolete for large rent machines, owing to the difficulty of constructing., it sufficiently strong, as the cores require to be laminated, Ilesides this. the number of poles which can be employed is limited, and when closely crowded they react injuriously upon each other.
Di.sl,•-Armaturea.—'I'hese may be divided into two classes : (a) Those in which a number of independent coils wound on bobbins, either with or without iron cores, are placed side by side in a circle, and revolve under the influence of a number of poles of successively opposite polarity. This type is specially adapted for the generation of alternating currents, and has been successfully employed by Wilde. Holmes, Sietnens—who used iron cores for the coils— and in the more modern maellines of Mordey and Ferranti (see below), in which iron cores are discarded. (b) Those in which the cores overlap a cf mnsiderable angle of the periphery, as shown in Fig. 78 (sec below), which represents the arrangement adopted in the Desroziers machine. Similar• arrangements have been •elopted by Edison, Pacinolti, Ayrton and Perry, Jehl and Rupp, and inure recently by Fritsche. the latter machine being known as the " wheel dynamo,•' on account of its peculiar• shape. (For practical examples of machines employing these various types of armatures see below.) In general, care must be taken to reduce the length of an armature conductor as much as possible, in order to reduce the in ternal resistance of the machine, to overcome which involves the consumption of power. In some machines the low resistance of the armature is the very basis of its regulating properties, as, for instance, in the shunt-machine (see below), Theoretically the form of armature re quiring the smallest length of wire for a given surface of magnetic induction is the circle. This has been carried out in the Thomson-Houston inac•hine (old type), and is also followed in the cores of some armatures of the ring type. But the advantages gained are not_ commen surate with the difficulties encountered in construction. and the rectangular form of section is now generally adopted. Only the best quality of copper with the lowest resistance should be employed, for reasons shnilar to those stated above.
The lnaticv of armature sections or coils to be employed varies considerably with different conductors; their number should, however, never he so small as to cause an appreciable fluc tuation in the strength of the current. Armatures should be designed so as to avoid excessive heating in the conductors; on account of the constant ventilation to which they are subjected they are capable of carrying a fa• heavier current limn couduc•tors placed in moldings, and without access to the circulating air. Thus, while in the latter case acnrrent of 1,000 amperes per sq. in. of copper would be a safe limit. in a well-ventilated armature a current density of 2,500 amperes per sq. in. is permissible. According to Ayrton and Perry, the permissible continuous output of a machine is a maximum when the thickness of the winding on the armature is such that the magnetic resistance of the air-space occupied by the winding on the armature is equal to the resistance of the rest of the magnetic circuit. Modern practice points to the following proportions in ring-armatures: The thickness of external armature winding is from 7 to 11 per cent of the diameter of the iron core, and in drum-armatures from 9 to 13 per cent. (For• actual windings, etc., adopted, see description of machines below.) Open and Closed Coil-Armaiures.—Aecording to the nature of their winding, and their connection with the commutator, armatures are divided into open and closed coil types. In the open coil type, of which the Brush dynamo is an example, the opposite coils are connected together and joined to two commutator segments, and form an independent circuit, there being an open cirrua between them and the remaining coils. In the closed circuit type (see Fig. 2) such as the Gramme and Siemens drum-type. each coil, besides being connected to the com mutator-strips is connected direetly to its neighbor. and forms one continuous winding, the coils forming a closed circuit. The former construction allows of the cutting out of circuit of those armature-coils which are not doing useful work when out of the influence of the mag netic field. This serves to reduce the internal resistance of the machine, a..d to increase its efficiency somewhat. Open-coil machines are used almost exclusively for enlist ructions in which it is desired to secure high potentials rather than heavy currents, us in arc-light machines.