This is the most common oil temperature gauge used for multi-engine aircraft of the war. It used a standardized size, combined with a scale in Centigrade, allowing it to be an easily replaceable and scavengable instrument. The B-17 used two Type A-24s. The left engines used one, and the right used one. The L and R designations on each gauge were used for engines on each wing, rather than left wing right wing. This allowed for a single gauge to be useable for both two and four engine aircraft.
As with most of the instruments installed in the cockpit, the A-24 was painted with UV and glow-in-the-dark paint. With flourescent lights on, neither was needed. These lights were only on for a short period, and rarely during a mission. With the UV lights on, the cockpits were much darker than with flourescent, but with the instruments painted properly, everything was still clearly visible to the pilot and co-pilot. The downside of UV, was that it still allowed a faint purple to be seen coming from the cockpit. With all lights off, the paint dims, however it still faintly glows for a long time. With all lights off, almost no light exits the cockpit.
The Type A-24 is a fairly simple instrument in theory. Two coils and two magnets, with two inputs and a cathode. However, the A-24 uses a more complex circuit, to provide better results. It has two terminals, designated to be + and -. It then has two other terminals, which control the readings. These four terminals, are all interconnected through a network of coils, allowing the remote electronic thermometers to show accurate readings.
A simple search on the internet, does not return any information regarding the Type A-24, and therefor no schematics are readily available. I disassembled gauge number 35785 and drew up my own schematic, based on the physical wiring. My first draft looked like this, as I was drawing as I was going, hence the mess:
From this schematic, and referring back to the physical wiring, I was able to draw up a final schematic which accurately shows the wiring within a Type A-24:
As you can see, it employs 12 different coils, four of which are rotary, and connected to the indicator needles. The Rear View shows which connection pins/wires are which from the schematic. Based on this schematic, pins 1 and 3 would be negative and positive, respectively, leaving pins 2 and 4 as control pins. Pin 4 would be biased negative outside of the instrument, allowing for a positive voltage on either 2 or 4 to move the appropriate needle. This positive voltage would be provided from the electronic thermometer, which would output different positive voltages depending on it's temperature in degrees Celcius.
While disassembling number 35785, I took a few pictures for future reference. Some are blurry, due to low light condition in my workshop.
More information will be added, when it becomes available.