Top of the horn, adjuster screw arrowed

Internals of the upper part. When the coils are energised they attract the iron disc to move the diaphragm. The operating pin is bearing on the operating lever, which has the moving contact attached to it. There is a piece of insulating material on top of the lever in this horn to fully isolate the electrical circuit from the casing. The pin pushes the operating lever, with its moving contact, away from the fixed contact to break the circuit and release the diaphragm. That allows the contacts to close again, repeating the cycle. This half of the casing is steel to form an efficient magnetic circuit with the coil armatures and diaphragm.

The 'crinkle' in the diaphragm has the effect of making it 'snap' between the operated and released positions (like a 'pop-o-matic dice shaker'), which greatly amplifies the sound the horn makes. This half of the casing is alloy.