You are correct. These speakers came right after horns. They were popular in 1927-28. Speakers are high impedance. They were operated directly off an output tube or through a large value cap.
Voltage through a coil changes the magnetic field causing the cone to vibrate. It should be adjusted so the reed driving the cone is centered and not touching either side of the metal. The gap should be small for max output but if it touches volume is reduced and distortion will be herd.
Norm
:I have a speaker in this "His masters voice" 28-V and it has no field coil and only 2 leads going to it and doesn't have a permanent magnet as they didn't have them yet I believe. The radio is 1920's and the speaker has a black metal horseshoe shaped piece on the back?? Anyone know a. What type of speaker is this and b. how it works? My guess is it is the transition between the horn (acoustic speaker) and the electromagnetic speaker? The cone doesn't move when you push it is made of jade green ribbed plastic and the center is brass with a solder joint in the center it says "Bosch Radio Speaker" "Made in USA, patented in Canada 1916, Model 612" Speaker is 8 inch. Thanx Brian. I can email pics of it if it helps..