A little more information on the solenoids that operate our starters.
A friend gave me a solenoid off a Bosch starter so I thought I would pull it apart to check the coils.
To my surprise both coils have the same number of turns.
The heavy coil I call the Grunt coil is wound on the bottom so it only uses a fraction of the wire that the outer coil does but its also a larger gauge.
The outer coil I call the holding coil is wound of a smaller gauge but the wire is also much longer because the circumference is greater therefore it will have more resistance and so draw less current.
The strength of a magnetic coil is dependent on the current x the number of turns (Ampere Turns)
Lets say the inner coil draws 40 Amps and the outer coil draws 10 Amps (pretty typical)
They both have the same number of turns therefore the relative strength is the ratio of the currents.
The inner coil is 4x as strong as the outer. (does 4 x the work)
Once the solenoid is pulled in the inner coil drops to zero current because its shorted out by the main contacts and just the holding coil holds everything in place.
Then interestingly when you take your finger off the button you probably assume the current is turned off, but no, now both coils are effectively in series. As they have the same number of turns the strengths will be the same.
BUT now one coil has the current reversed so the field is equal but opposite, in other words the fields cancel out so the solenoid lets everything go, that's pretty neat I reckon.
I didn't do a very good job of explaining that, I know
