Page:Electricity (1912) Kapp.djvu/200

196 end of the wire diametrically opposite is taken across the face of the drum. It is obviously impossible to take it straight across, for on the one side there is the shaft in the way and on the other the commutator, the diameter of which may be half that of the drum or even more. Thus the space at both drum-heads is not free, and to take the connections across we must so shape them as to avoid these central obstructions and at the same time not interfere with each other. This is a purely mechanical problem, and has been solved in various ways. As it has only interest for the professional designer of dynamos the various solutions need not be detailed here, but it may be pointed out that the mechanical difficulties of arranging the end connections become less serious when drum winding is applied to a machine having more than two poles. In a four-pole machine the end connection need only span a quarter of the circumference; and in a six-pole machine only one-sixth. Thus to avoid commutator and shaft is an easy matter, whilst avoiding mutual interference of end connections is also easier, since their length is reduced and there are fewer of them crossing each other. This is one of the reasons why modern