The typical spiral core is shown in Fig. 9. The core is assembled either of a continuous strip of transformer steel wound in the form of a circular or elliptical cylinder or of a group of short strips assembled to produce the same elliptical-shaped core. By using this construction the core flux always follows along the grain of the iron. Cold-rolled steel of high silicon content enables the designer to use higher operating flux densities with lower loss per kg. The higher flux density reduces the weight per kVA.