This is the power supply that came with my Apple MacBook Pro. It’s got the fancy magnetic connector at the other end, but the transformer unit itself is surprisingly large. In fact, it’s very carefully designed to make it impossible to plug it into any mains socket in any British institutional building, where the sockets are invariably in the wainscoting — as, for example, here (Downing College, Cambridge):

It fell to James Cridland to suggest the obvious workaround — to remove the three-pin plug and use a standard mains lead (the kind one uses to charge the batteries of many digital devices).

Obvious when you think about it — or rather when he suggested it. But it means lugging around another piece of wire. Sigh.