Graham. wrote:

Only joking, but it was similar, suspended in a metal hoop on four springs.
it had a Bakelite base housing the "induction coil" transformer, I remember
being surprised that the core was made up of many parallel iron wires.

As a child I discovered that the carbon mike inserts used by the GPO
would get very hot if subject to a high voltage. This was during
attempts to get a usable audio link between our house and my mate's
using very thin blasting wire nicked from the quarry and a lot of
enormous batteries nicked from the railway.

Bill