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When Ford made the change from gas headlights to electric on the 1915* Model T's, the improvement was in ease of operation. All you had to do to turn on the lights was push a switch on the dash. But was the lighting any better? Under the best of circumstances magneto lights are marginally adequate. In town, with the help of street lights, they are fine. On a familiar, straight country road where the driver knows the way, they're sort of adequate. On an unfamiliar, dark country road, especially with curves, they're dangerous. Their brightness depends on engine speed. At high speed they're bright enough, but you outrun their limited range. When you have to slow down for a turn, they go dim. That doesn't mean slightly dim. It means you can't see where you're going. I once had to drive on an unfamiliar dark country road by magneto light. I won't do it again.
*Mag lights were standard 1915-1918. Battery lights came with the electric starter in 1919.

The magneto lighting system introduced on 1915 Fords was quite simple. It was one circuit containing a switch and two bulbs connected in series. That means that if one bulb fails the circuit is broken and both bulbs go out. For this reason it's wise to carry spare bulbs in the car. More about that later.

The diagram shows us that a wire from the magneto contact goes to the magneto terminal on the coil cox and is the only source of current for the ignition, horn, and lights. There is no wire connected to the battery terminal because that's how the car came from the factory. But drivers who couldn't pull the crank fast enough to generate enough current to fire the plugs could install a battery to fire the plugs for them, and a slow pull would start the car. If a battery is used in this system it powers only the ignition. The horn and the lights remain powered by the magneto and will work only when the car is running.

These two views of a 1915 radiator show the tube that protects the wire from being damaged by the fan. A wire from the switch goes to the right headlamp, and from there a wire goes through the tube to the left headlamp.

The wire coming from the right headlamp goes up to the left headlamp, and a wire from there comes down to ground at the radiator stud. Adding a star washer assures a good electrical contact.

A magneto headlamp bulb has a single filament, two contacts, and equal pins. The base only holds the bulb in the socket. Neither is part of the circuit. If a magneto bulb is marked for voltage it will say 9 volts.

Filaments vary. This one is in an actual 9 volt bulb.

This is the filament in a General Electric 1134 bulb marked 6-8 volts. While not intended as magneto bulbs these don't burn out as quickly as I expected. My experience has been that they do burn out eventually, but last a long time before they go.

Bulbs that can be used in mag lights vary in shape. This one is unmarked, but works.

I have found mag bulbs and suitable substitutes at auctions and swap meets.

Often you find bulbs in original packaging that identifies what they are.

Another look at a GE Eveready #1134 bulb. 6-8 v, 32 CP.

These Amalco #1138 bulbs are 9v 27 CP.

Crest #1138 bulbs, also 9v 27 CP.

The style of the generic cars on this Tung-Sol box tells us these bulbs were most likely made in the early forties.

This is the bulb currently sold by Model T parts dealers as a "Magneto Headlight Bulb". If all you want to do is take a picture of your car with the lights on, it will work for that. If you want to drive by mag lights, it's $4.25 wasted. The only marking on it, 24v, tells the story. It may last forever, but it will be too dim to make enough light for driving.

There is a problem that sometimes happens when you use bulbs that are a hundred years old or more. Over the decades the glue holding the glass in the metal base has failed. When you insert the base in the socket and turn the bulb to lock the pins, the glass twists in the base and the bulb is wrecked. I once had a bulb go out when I was traveling. "No problem," I thought. "I have plenty of spare bulbs." But all seven of the spares twisted and wrecked. Of course the auto parts store across the street had no bulbs that could be used for mag lights, so I camped in a parking lot until daylight. To avoid this problem in the future I run a bead of glue around each bulb to connect the glass with the base. The one in the picture is rather sloppy, but with practice I can do it more neatly.

There are various tools for removing headlight rims to change bulbs.
This one has a steel ring that clamps on the rim and has four holes in the ring that fit onto the rivets in the rim.
It comes apart at the hinge for convenient storage under the seat.

Bulbs with two contacts and even pins (BA15d base) are not all suitable for use in magneto head lamps. If they are suitable and properly matched they will be bright, and equally so.	If bulbs with the right base are not a match in other ways, one may be bright and the other dim. In some cases, an unmatched bulb is so dim that it doesn't even light up. In this picture current is passing through both bulbs, but only one of them produces any light.
A simple tester for mag bulbs is just a switch and two #1177 sockets wired in series.	Two bulbs lit brightly and equally are good to go as a matching pair.

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