DIRECT-CURRENT DYNAMOS AND MOTORS. 
Any departure from a circular cross-section is objectionable 
for the following reasons: : 
1. The circle has the least circumference for a givén area, 
even the perimeter of an equivalent square being: about 
12 per cent. longer; and a rectangle with one side three 
times the length of the other, has a perimeter nearly 30 
per cent. greater than that of a circle of equal area. 
2. It is much easier to make a cylinder, since the pattern 
and core itself can be turned in a lathe. 
3. Cylindrical spools for the coils are more easily made than 
elliptical or rectangular ones. 
4, The operation of winding is much more difficult with a 
rectangular or elliptical core, since the strain on the wire 
is very unsteady. 
5. A rectangular, or even an elliptical, core is much more 
likely to cause a short-circuit between the turns of wire, 
since they are forced together at certain points and are 
liable to cut through the insulation. | 
Similar arguments apply to a core having a curved axis, 
that is, those having ring or bow-shaped field-magnet 
cores. These latter are far more difficult to wind, and 
possess little or no compensating advantage. The short- 
ness of magnetic circuit which is claimed for them is very 
doubtful, for the reason that the actual length of core re- 
quired for equal average depth of winding is greater than 
if it were straight, since the winding on the inner side of the 
curve is higher than on the outer side. It often happens 
that, in the design of dynamos, it is apparently desirable 
to adopt a core which is not circular in section, or has a 
curved axis; but it is better to change the entire design in 
order to avoid these forms, except, perhaps, for a special 
machine to fit in a certain limited space. 
In the case of multipolar magnets of the ring type shown in 
Figs. 21 and 22, the inner ends of the cores form the pole 
faces; and since it is desirable for the latter to be rectan-: 
gular rather than circular, in order to have as much sur- 
face as possible, it is a common practice to make the cores 
rectangular also. But when the pole faces are extended 
