MTFCA Forum

From reading Model T lore you might think magneto headlamps burn out at a furious rate. But when I had a bulb go out last week that pair had been in the car for at least three years. I don't do much night driving, but I seldom shut off the lights. When the car is running the lights are on.

Mag lights being wired in series, like old time Christmas tree lights, when one dies they both go out. Being wired in series also means the bulbs must be the same kind, with the same filament, voltage, resistance, etc.

Matching bulbs light equally.

If the bulbs don't match one will be bright and the other dim.

One may be so dim that it doesn't even light.

So to be sure the spares I carry in the car are matching pairs, I made a simple tester. It's just a switch and a pair of 1176 sockets wired in series. I had to buy the sockets online, of course.

If two bulbs light equally I can pack them as a pair.

That sure a large battery just for testing light bulbs.

That sure a large battery just for testing light bulbs.

It comes in handy for running the car.

Dad put the mag lights back in the 12 in 1967 and not used much in the past 50 years or so we have never replaced a bulb. When I need them they always light. Im still waiting to burn one ,and have 50 odd new ones, maybe that keeps the old ones going. This is the first time I herd about matching I'll have to look in to this Cheers Colin

The original bulbs were 9 volts, in series 18 volts resistance and the mag rarely reached that voltage at normal driving. Most bulbs today are 6 volts and pared will only attain a 12 volt resistance, still OK if you don't rev it too high. For an added safety a third 6 volt bulb can be wired in series into the circuit for a total of 18 volts resistance. The bulb can be hidden under somewhere or displayed for additional lighting purposes.