Project In Progress. 
History of this set
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.
Apparently around 1 million of these were made! I wonder how many are left?
Part of my collection
