I have a question on the wiring for magneto headlights - it appears that the headlight switch is feeds from the mag and then feeds the one contact of the passenger hl bulb, then the other contact. Connects to the drivers hl. And then another wire from the second contact on drivers hl goes to ground. Is this correct?.
Yes, the bulbs are connected in series. If one burns out, they both go out.
For your convenience, I took photos of my dad's 1917 a few years ago - see the thread below.
Note that from the factory the ground wire was originally soldered to the radiator, not looped around the radiator shell stud as shown in the photos.
Rather than soldering a wire to the radiator as Erik mentioned, I suspect most people ground their mag lights the same way he and I do.
For the purists:
Here is the ground wire on the 1917 Rip Van Winkle Touring. There usually is a reason for everything that Ford Motor Co. did but who knows why they soldered it to a radiator fin?
Also below is the firewall - note the two staples that hold the lamp wire.
The bulbs were originally 9 volts and would handle the 18 - 20 volts the mag would put out. 9 volt bulbs are no longer available 6 - 8 volt bulbs can be used but may burn out with high RPM, 12 volt bulbs can be used but will be dim. It is difficult to find double contact single filament bulbs but that is what is required. You can not use bulbs with the socket as a ground. A modification that will work satisfactorily is to wire three 6 volt bulbs in series and hide the third light. This will offer the magneto the correct resistance.
Actually ANY arrangement that allows any bulb to get more than its design voltage will result in very very short bulb life. What was not known nor understood by Ford but well known now was that tungsten filament bulb life is very sensitive to voltage. The accepted formula is to take the ratio of the applied voltage to the design voltage and raise it to the 12th power and you will then have a multiplier to predict the expected multiple of bulb actual life compared to its design life.
So lets start with a pair of 9V bulbs (which I agree is what they used since I have looked at this in the archives). Since the bulbs are in series and the normal high end of the magneto is taken as 28V then each bulb will get 14V applied to it when you are cruising down a smooth road at night at speed of 30 Mph or more. This means you have applied 14/9 = 1.555 times the normal bulb design voltage. 1.555 raised to the 12th power is a shade over 200. So you will get 1/200th of the bulbs normal life or about .5% of its normal life. Thus if the bulb is say a 300 hour bulb then on average you will get about 1.5 hours of life out of it. I kid you not. What you do need to understand is that during the 15/16 era you were not likely to be cruising at high rpm and 30 MPH since the roads were not that good thus bulb life was short for sure but not nearly as short as it is today with our high speed cruising being the norm and high magneto output being sustained at near max out.
Ford was convinced that the bulb makers were not making quality bulbs so Ford went so far as to design the bulb filament themselves and force the vendors to make it and they gladly did that which quickly proved Ford bulbs didn't do any better.
So what to do? If you intend to drive cars at night I strongly suggest you wire the headlight sockets in parallel and put a rechargeable battery (6 or 12) under your seat and then operate with 6 or 12v lights. Only a very careful observation will give away your secret since this does not mean a huge modification to the car wiring but does afford a huge modification in the reliability of your headlights. ALSO - add a very brite tail light too. Magneto operated bulbs in series are a sure "lights out" event and it won't take very long.
Hope this helps.
I may eventually change to battery power for the little night driving I do with my 1915, but for now I'm running the original magneto light setup. Being unschooled in the technical in formation John has cited, I can offer only my limited personal experience. I'm using #1142 twelve volt bulbs. These are not designed for headlight use, but I find them adequate for night driving on familiar roads around home. They have the advantages of easy availability and low cost. I never turn off the lights. When the car is running, day or night, they're on. So far I've never had one burn out. Earlier this year I tried the "Magneto Headlight Bulbs" from Lang's. It turned out that they're 24 volt bulbs. I understand the intention of using a bulb that's unlikely to be burned out by magneto voltage, but the problem is that they're so dim you might as well have no lights at all. After one scary drive with them, I went back to the twelve volt bulbs.