(a) Theory dictates that if iron is employed in armatures, it must be slit or laminated, so as to prevent the generation of Foucault currents. Such iron cores should be structurally divided in planes normal to the circuits round which electromotive force is induced; or should be divided in planes parallel to the lines of force and to the direction of the motion. Cores built up of varnished iron wire, or of thin disks of sheet - iron separated by varnish, asbestos paper, or mica, partially realize the required condition.

(b) Armature cores should be so arranged that the direction of polarity of their magnetization is never abruptly reversed during their rotation. If this precaution is neglected, the cores will be heated.

Armature Coils

(a) All needless resistance should be avoided in armature coils, as hurtful to the efficiency of the machine. The wires should be as short and thick as is consistent with obtaining the requisite electromotive force, without requiring an undue speed of driving.

(b) The wire should be of the best electric conductivity. The conductivity of good copper is so nearly equal to that of silver (over 96 per cent.), that it is not worth while to use silver wires in the . armature coils of dynamos.

(c) In cases where copper rods or strips are used instead of wires, care must be taken to avoid Foucault currents by laminating such conductors, or slitting them in planes parallel to the electromotive force; i.e. in planes perpendicular to the lines of force and to the direction of the rotation.

(d) In dynamos of the first class, when used to generate currents in one direction, since the currents generated in the coils are doing half their motion inverse to those generated during the other half of their motion, a commutator or collector of some kind must be used. In any single coil without a commutator, the alternate currents would be generated in successive revolutions, if the coil were destitute of self - induction currents. But if by the addition of a simple split - tube commutator, the alternate halves of these currents are reversed, so as to rectify their direction through the rest of the circuit, the resultant currents will not be continuous, but will be of one sign only, there being 2 currents generated during each revolution of the coil. If 2 coils are used at right angles to each other's planes, so that one comes into the position of best action, while the other is in the position of least action (one being normal to the lines of force when the other is parallel to them), and their actions be superposed, the result will be to give a current which is continuous but not steady, having 4 slight undulations per revolution.

If any larger number of separate coils is used, and their effects, occurring at regular intervals, be superposed, a similar curve will be obtained, but with summits proportionately more numerous and less elevated. When the number of coils used is very great, and the overlappings of the curves are still more complete, the row of summits will form practically a straight line, or the whole current will be practically constant.

(e) The rotating armature coils ought to be divided into a large number of sections, each coming in regular succession into the position of best action.

(f) If these sections, or coils, are independent of each other, each coil, or diametrical pair of coils, must have its own commutator. If they are not independent, but are wound on in continuous connection all round the armature, a collector is needed, consisting of parallel metallic bars as numerous as the sections, each bar communicating with the end of one section and the beginning of the next.

(g) In any case, the connections of such sections and of the commutators or collectors should be symmetrical round the axis; if not, the induction will be unequal in the parts that successively occupy the same positions with respect to the field - magnets, giving rise to inequalities in the electromotive force, sparking at the commutator or collector, and other irregularities.

(A) Where the coils are working in series, it is advantageous to arrange the commutator to cut out the coil that is in the position of least action, as the circuit is thereby relieved of the resistance of an idle coil. But no such coil should be short - circuited to cut it out. Where the coils are working in parallel, cutting out an idle coil increases the resistance, but may be advisable to prevent heating from waste currents traversing it from the active coils.

(i) In the case of pole - armatures, the coils should be wound on the poles rather than on the middles of the projecting cores; since the variations in the induced magnetism are most effective at or near the poles.

(j) Since it is impossible to reduce the resistance of the armature coils to zero, it is impossible to prevent heat being developed in those coils during their rotation; hence it is advisable that the coils should be wound with air spaces in some way between them, that they may be cooled by ventilation.

(k) The insulation of the armature coils should be ensured with particular care, and should be carried out as far as possible with mica and asbestos, or other materials not liable to be melted, if the armature coils become heated.