Left side is Model T Trembler Coil. The SPDT is where the soft iron contact that is magnetically pulled down by the iron core transformer primary by 6 volt DC battery or 7 volt AC magnito (the trembler) makes and breaks contact similar to a school bell. Best Internet video I found on this is:
https://www.youtube.com/watch?v=hKRv-E_HC2Y
We use this high voltage secondary to smoke out a wire break in insulated cable. If two wire cable twist ends together to make contact loop circuit. If single cable add second cable to make loop circuit and connect to coil secondary. The break area of cable arcs and heat, smoke, and hot to touch shows where break gap is (shown as burning bulb in simulator).
Modern methods today are non-contact AC Voltage detector and send hot line AC signal down each wire and walk down the wire to see where current stops.
