Magneto, or battery?
Steve, You could always hold it up to your forehead like the Great Carnac to tell which it is!
Steve, It could be both. You must open it to be sure. The late magneto horns had only one terminal.
That horn is Heinze Electric Co., magneto horn.
OK, I'll have to open it up when I refinish it. How are the magneto and battery guts different?
I have the same horn on my 17. Mag for sure.
Before you open it up:
Apply six volts DC.
If it buzzes, then it's battery.
If it makes a single "click" or doesn't sound at all, then it's most likely magneto, unless it's a mis-adjusted battery horn.
here is how a magneto horn looks inside.
The MAG horn on my '17 has two terminals and looks nothing like that.
There were several manufacturers of magneto horns, beginning with Bridgeport Brass in 1915. HECO came in a bit later, as did another company (possibly Briggs and Stratton). The HECO magneto horn is characterized by the semi-conical shape of the back of the horn. The other style is characterized by having a back shaped like a top hat. It is cylindrical in the middle, extending up about 1 inch, sitting on top of a slightly raised rim the diameter of the rear of the horn.
The earlier magneto horns had two contacts, one from the magneto and the other to a ground. At the moment, I do not recall where the ground wire on this type of horn actually went to. In 1915-16 Ford ran ground wires to some interesting location. For example, the magneto headlight ground wire ran to the steel support bar in the radiator, and was soldered to that bar on the left hand side.
A photo image of a top hat style magneto horn with two contacts is shown below;
The horn Steve Jeffs has pictured is definitely a magneto horn.
Later on, The Ford engineers realized that they could save money by grounding the magneto horn internally, reducing the number of contacts from two to one. The entire horn then had to be grounded to the car through its mounting bracket. If you look at the horn Steve has pictured, it is attached to a simple bracket with two mounting holes. The left hand hole in the mounting bracket was attached to the dash using the lower right steering column to dash mounting bolt and nut, and the other hole was attached by a separate carriage bolt.
By attaching the horn to a steering column mounting bolt, the horn was grounded to the chassis through the mounting bolt, the steering column, and by the steering column to the frame.
I think the dent in the projector is a tuning dent, might not sound right if you take it out!
For 15/16 the ground was under the pushbutton via a thin copper strap that was captive between the horn button frame and the steering column thus the ground was to the steering column. The wiring was from mag connection at the coil box to one connection on the horn then the other horn connection was to a wire that ran up the steering column to be grounded to the column by the pushbutton. When the "combo" light switch and horn button showed up in late 1917 (?) the wiring was changed and the magneto horn only needed one connection with the frame of it grounded for the other. In this era the wiring was from magneto hot connection up to the combo switch and then the horn button connected that magneto wire to another wire that ran down to the horn. The combo switch connected that same mag wire to one of 2 wires running down to the dimmer coil which was mounted on the back side of the dash. The "common" connection of that dimmer coil was then to the headlamps. What is interesting with the magneto horn from this era was that they still had 2 terminal mag horns left from the 15/16 era and they had to ground one of the connections. What they did was clever. They simply removed the top insulator fiber washer from one of the connections and put the nut back on to thus make that connection to the horn housing and just like that you had a single hot connection type mag horn since the other horn connection still had the insulating washer. No need to get inside the horn for that.
Easiest way to spot a mag horn is look down into the snout of it and see if there is an adjuster screw in the center of the diaphragm. If YES then it is a magneto horn. Battery horn had nothing attached to the center of the diaphragm on either side of it (internal or external).
Lots of good stuff in this thread, thankyou all!