1915 single contact headlights

[jyancho]

I’m looking at the model T judging guidelines, 4th edition and it says under 1915 headlights (180) magneto style, single contact.... my question is were these lights wired in parallel? If they were single contact it wouldn’t be possible to wire them in series as the wiring diagram indicates?

[Colin Mavins]

My 1912 has accessory 1915 mag head lamp conversion ,one wire from the switch to the left lamp and one wire from the left lamp to the right lamp and if memory serves there is a ground wire on both , the 12 brass lamps have the lamp fork lock bolt on the inside of the lamp and the ground wires are connected to them. The lamp socket takes two wire each one power one ground . Yes the bulb is a single contact. Cheers Colin

[jyancho]

Thanks Collin. Sounds like you’re describing double contact lights in series. The problem I’m having is the judging guide says they should be single contact. A single contact bulb has one side of the filament going to ground through the base and the other end goes to the single contact. So, there’s no way to wire single contact bulbs in series. The wiring diagram says 1915 should be wired in series as you describe with your accessory lights, which I bet are double contact.

[Steve Jelf]

The guidelines are wrong. That's probably why the sixth edition doesn't mention number of contacts or location of pins, but says simply "large magneto bulbs". 1915 lights are wired in series, and mag bulbs have two contacts. Current goes in one contact and out the other. The bulb base is not part of the circuit.

[Piewagon]

As Steve and others have said the Judging Guidelines are wrong if they say that the 1915 bulb is a single contact. I think what they might have meant was that it is a single filament bulb but separate ends of that filament are connected to separate contacts at the base and there is nothing electrical connected to the outer brass base part as is typical with a single contact bulb. Mag bulbs were rated at 9 volts each and if you rev up the motor a pair of them would be fully lit at 18VAC out of the magneto but of course a healthy magneto puts out more than that and that is why mag bulbs quickly burn out. Tungsten filament bulbs do NOT like to be operated beyond their design voltage which is in truth a single voltage level and not some wide variable voltage and Ford found this out the hard way. Ford blamed the bulb makers at first and then went so far as to demand that the bulbs be manufactured to FORD's design including Ford's filament design. The bulb makers happily supplied these. When Ford's bulbs failed just as quick as all the others then they went to dimmer coils and efforts to stabilize the voltage to prevent quick failure of the mag bulbs. They had limited success at that and really didn't have good lights until the system was converted to run off the battery. You may find bulbs marked 6-8 volts but those were made when the thinking was that you could run them off that range of voltage but in truth only one voltage is used for the actual design and then the life is typically derated at all other higher voltages and increased life happens with lower voltages. The amount of increase or decrease in life is dramatic with small changes in bulb voltage.

If you intend to drive at night with magneto bulbs you ought to limit your travel to the length of your driveway! On your driver T use battery powered lamps wired in parallel and put a battery in the car of modest size to give you the needed hours of driving that you expect to do in one night then connect the battery to a charger the minute you get home. Hope this helps.

[DanTreace]

And here's one of those original Mazda 2 contact terminal magneto bulbs. These are single CP, this one 16 c.p. , 12-16v.

And comparison with typical electric 2 contact terminal bulb, these are usually dual filament, so CP on the lower one is 21c.p and 2c.p.
The electric DC bulbs are normally sporting a solder point at the top of the brass base, indicating the base is the ground.

As John posted, most of the magneto bulbs got too much AC from the magneto, and look browned out and blown, just like the one in top of photo

[Steve Jelf]

If you intend to drive at night with magneto bulbs you ought to limit your travel to the length of your driveway!

I agree with John on that, with an exception. If you're in a city or town where there are street lights to help out, it's OK. But I have also driven by mag lights on an unfamiliar country road. Once. I will not do that again. It's just too scary and dangerous. I almost wrecked. For the lights to get bright enough to be of any use, you have to be going fast enough to outrun them.

I have found that the 6-8 v bulbs last better than I expected, perhaps because I don't do much night driving. But I do carry at least half a dozen spares, just in case.

I wouldn't have done this without a bit of early morning daylight.

[Mark Gregush]

Re brightness of mag lights; That fits what I have read. When they came to a corner they would shift into low and rev the engine up so they could see, of course not too high so as not to burn the bulbs out.

[Altair]

The simple terms are single filament with double contact to wire them in series. In the day they were 9 volt bulbs but they are not available any more. 6 volts are at high risk of burning out and 12 volts are too dim. What can be done to reduce the risk of burning out 6 volt lamps is to wire a third bulb in the system and place it out of sight, just a means to use the available current. The three 6 volt lamps in series will accept 18 volts and substantially reduce the risk of burn out.

[Novice]

There are a number of ways to regulate the magneto output voltage to the head lights to maintain a fairly constant voltage / brightness. there was a article about using a standard model T spark coil. coil/core to make a magnetic magneto voltage regulator. or go electronic rectifier / regulator.
see magneto voltage control pg 86 Tinkerin tips. Volume 1 by Ted Aschman