A solenoid has no polarity (Pos earth, Neg earth) but is voltage specific. The solenoids with one small terminal rely on the mounting tabs to grounded. Supply battery power to the small terminal will energise the solenoid and connect the two large terminals (On some solenoids, the contacts are normally closed amd supplying power will break the contacts). On solenoids with two small terminals, the body is insulated and the two small terminals need to be wired to battery (usually through a switch) and earth, which will then connect the two large terminals. Solenoids are available in a range of voltages (6v, 12v, 24v) and configurations (Grounded body, two terminal, contacts open or contacts closed). Solenoids with the push button like the original 01A-11450 A are becoming hard to source. All the button does is manually close the contacts, which is handy to turn the starter when you're in the engine bay doing a tune up..
