The link above is just one of a dozen or more very helpful websites for Ford 8n tractors.
The original 8n tractor had a six volt battery with positive ground (this is different than most automobiles that have a negative ground). The starter solenoid is energized via the ignition switch and the large starter button in front of the gear shift, the small wires going to the solenoid energize a coil which "pulls in" and completes a circuit that then provides 6 volts directly to the starter motor through a large, heavy gauge wire. The smaller wires connect to the two terminals on the top of the solenoid and the heavy wire goes to the larger post on the solenoid and originates from the negative post of the battery... Hope this helps. There are many, many schematics and pictures available on the "net" The Ford 8n community is very large and extremely helpful....Good Luck