The Snakes Alive CMP Wiring Project

[Jacques Reed]

Hi Mariano,

The direction of the magnetic field in a solenoid is determined by the current flow in it. if you swap the positive and negative terminals in a solenoid the direction of the magnetic field changes direction 180 degrees.

So, if a positive earthed solenoid (early Ford) was replaced with a negative earthed (later) one the magnetic field, and thus the force, on the armature, the centre part that closes the contacts between the battery and the starter, would be reversed.
This would cause the force on the armature to be away from the contacts and thus no closing of them to supply current to the starter would occur.

Search "Right hand Rule for Solenoids" for more info.

Also, as current flow by convention is positive to negative, perhaps "voltage activated" refers to a positive voltage applied to the solenoid in a negatively earthed one.

Hope this helps.

Cheers,