in this figure are represented as they would be seen from one of the ends of the machine after the plates at the end and the coils had been removed.

It will also be seen that the space between the two polar pieces is larger in the centre than towards the extremities of the armature, the length of the grooves diminishing towards the centre. According to Weston, a greater regularity of current is ensured by this arrangement, as the induction of the armature coils is effected from the centre towards the extremities, and vice versa, instead of taking place simultaneously along the entire length of one of the armature elements. The grooves in the polar pieces are meant to prevent the production, in the iron mass, of Foucault's currents, and further to facilitate the cooling, by ventilation, of the polar pieces and of the armature.

Fig. 55.-Polar pieces of the armature.

This latter has quite a particular construction. The cylinder on which the wire is wound, instead of being of

one single piece, is formed of a number of sheet-iron wheels of the form represented in Fig. 56. These wheels have sixteen teeth each, are fastened on the shaft and separated by discs, of which there are thirty-six. At both extremities two solid pieces of the same diameter as the discs are fastened to the shaft to keep the toothed wheels and discs in their places. The armature has now the form of a

Fig. 56.-Armature-wheel.