As Joestue said the ideal core is a strip of grain oriented steel wound through the coil bobbin so the flux is along the grain axis. This is costly to manufacture and the usual compromise is a wound strip core cut in half and the faces polished. When tightly clamped together this comes close to the ideal but with some effects from the cut.
These are usually assembled in pairs side by side giving an effective "E" type construction.
To justify this more costly method it is invariably used with the best grade core materials. Mainly used for higher frequency ( 400Hz aircraft) equipment or for compact high performance power frequency devices. If made the same size as a standard unit the efficiency will be higher but it comes with a fair cost penalty. If the efficiency needs to be high and cost is not a factor then you may occasionally find them but most commercial units now seem to get by with a new type of conventional core material that has replaced the old silicon iron punchings ( likely a nickel alloy as it seems soft more like mumetal).
Flux[ Parent ]
non-aligned transformer iron has about 30% less loss in the rolling direction, (i'm no engineer so i don't remember the proper term here) By lengthening the core in this direction you can add a disproportional amount of copper. but its a 2-4% increase.
Another point to bring up is large transformers have a core built up from 15 or more sizes of steel, the core has about a 90% or better fill factor in a circular area, so the copper loss is 3.1415/4 or 78% that of a square form.[ Parent ]
