And what's it for? Voltage spikes maybe? It's just under the voltage
regulator on the firewall of a 1966 172, which, btw, is a wonderful flying
airplane, as I can attest from experience.

Whoever installed this device did a sloppy job because the wire was out of
the oversized terminal on the switch side. Anyway it's apparently a tiny
resistor wired to the starter switch on the solenoid and the other end of
the wire runs to the solenoid's hot terminal.

Curious.