I saw this on the AACA web site. In case you are interested it is for sale, I have no connection with the seller.
Regulator for magneto headlights?
How does they work? A coil that would give variable resistance as the frequency and voltage goes up so does the resistance (reactance I think it is when it's a coil)?
That way it will be an almost constant current regardless of speed.
I checked your profile but it did not say what type of Model Ts you have. The 1915-1916 cars that first had electric headlamps from the Ford factory had some issues. The headlamps were wired in series and powered off the magneto. As the engine speed increased the voltage from the magneto went up. The lights got brighter and probably hotter. And on bumpy roads – the failures would be more often. And when one light bulb failed it opened the electrical circuit so the other headlamp that still had a usable headlamp bulb now did not have electricity flowing to it. So it got really dark.
Ford began installing part number 8892X a Headlamp DIMMER RESISTANCE COIL and WIRES probably sometime during the 1917 production, based on Bruce’s (RIP) Price List of Parts. Note that the photos of the Jun 1917 Rip Van Winkle Ford do not appear to show a Dimmer Resistance Coil. But our wrecked Dec 1917 (1918 model year) car named “Donor”[bummer of a name for a car in the garage – could be prophetic of its future?] did have that on it’s engine side of the firewall. If others have a 1917-1920 ish non-starter car with the 8892X Headlamp Dimmer Resistance Coil mounted on the original firewall – would you please post a photo and let know what year and month your engine is? I suspect that the factory drawing would have been updated to include drilling some additional holes to mount the Headlamp Dimmer Resistance Coil. (Note when Ford stopped using the coil box with the switch on the front and went to the switch on the dash even for the non-starter cars – the blank off covering the hole for the amp meeter had the 8892X located behind the blank off. )
The photo of the Ford part #8892X was originally posted by Keith Townsend and Gary White added the arrow pointing to the part at the posting at:
For a description of how the Ford part works (and that may also be how the accessory part works – but without the additional “dim” position) see John Regan’s comment on the thread at: http://www.mtfca.com/discus/messages/29/29261.html?1178994052
Hap l9l5 cut off
Over the years, there has been a lot of misinformation posted on the forum regarding the changeover from the push pull light switch to the combination horn button/light switch. Also, Bruce's information on the subject is, in my opinion, not reliable.
The resistance coil appeared when the push pull light switch was removed from the dash and was replaced by the combination horn button light switch which was sometime during the 1918 model year, NOT the 1917 model year.
John Regan gives dates pertaining to the changeover below, but incorrectly states that the 1918 model year started October 1, 1917:
Here is another post where John Regan also provides accurate information but it is toward the bottom of the thread where he does so: