Spiral Core Transformer

Question

Presently, I am reading about different types of transformer. I am stuck on 'spiral core or wound core transformer'. I didn't find any good explanation about it in the textbook I am reading. Searched on the internet in hope to find more detail, but same thing, no good description.

I am providing the text from the book I am reading as follows:

In spiral core, spirals of strip steel are wound through the window of perforated coil. The resultant structure provides continuous magnetic path unobstructed by butt or lap joints hence the path of reluctance is reduced.

I don't understand the words in bold. It will be helpful if it is possible to provide a picture for better understanding.

Answer 1

This is a description of that is called "wound core" transformer. A single strip of magnetic material (silicon steel, for example) is wound into a core.

For example:

You can google butt joint and lap joint- a butt joint is just two flat edges pressed up against each other, a lap joint is stepped.

(image of E-I laminations below from this web page):

Seems pretty easy- certainly compared to stacking up dozens of E-I or E-E laminations (flipping the former each pair results in a kind of lap joint)- so why don't all metal core transformers use this?

You can see the obvious disadvantage- the wound core is topologically equivalent to a toroid- there's no way to slip an already-wound bobbin onto the core.