Early diaphragm Loudspeaker.    No B.B.C / G.P.O Stamp.  B.T.H = British Thompson-Houston Co Ltd  Working.

Apparently around 1 million of these were made! I wonder how many are left? I use it to listen to the one two or three valve sets in my collection. Basically it consists of a cast aluminium horn, which is connected to a bakelite base. Inside the bakelite base is a headphone earpiece 'beefed up' to take more current. This is coupled to the horn which distributes the sound. It has a knob underneath which adjusts the distance between the diaphragm and the coils which acts as a sensitivity/volume control.

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