In article m, David
Nebenzahl wrote in part:

Yes. All power transformers for low-frequency AC are basically the same:
an iron core of some type with windings around it. Small transformers,
like the ones in audio equipment, TVs, etc., generally use "E"-shaped
cores, with one winding around each outside leg of the "E".

Low frequency power transformers in this power class have "E-I" cores,
with E and I "laminations" alternating/interleaved. The "I pieces" are
adjacent to the 3 tips of the "E pieces". The alternation has E's
sandwiched between I's, and I's sandwiched between E's.

The "center leg" of the "E pieces" has width twice that of the 2 "outer
legs", that of the "backbone of the E", and that of the "I pieces".

Unlike what you said, both the primary and secondary windings are around
the "center leg of the E's".

If an E-I core is used for a transformer requiring some DC in the
primary (such as a "vertical output transformer" of a mostly-tube TV or
output transformer of a single-ended vacuum tube audio amplifier), then
there is likely need for a gap in the core. In that case, the E's are
stacked together pointing the same way, the I's are similarly stacked into
a rectangular block, and some layer of paper or cardboard or whatever is
typically used to space the I's from the E's. The windings are still
around the center leg of the E's.
2-lead fluorescent lamp ballasts (for lamps at most 22 watts in USA) and
"reactor" type ballasts for other lamps and "filter chokes" for some power
supply designs generally have similar E-I core with an "E stack" and an "I
stack" and a gap of some sort.

Lamp ballasts with more than 2 leads (excluding any used by starting
circuitry) and "neon sign transformers" (which are actually ballasts) get
different by using more complex core designs with "magnetic shunts".

- Don Klipstein )