Can someone explain to me why moving the third brush puts more current to the field to increase the rate of charge. I have an IH tractor and have noticed that the closer the third brush is to the ungrounded brush the higher the charge. Can moving the third brush cause the field current to be picked up when the magnetic field is stronger? I am sure this will be a very routine question for some electrical engineer.

It has to do with the way the armature is wound.
The farther you get away from any two adjacent segments on the commutator the less the output will be.
If you want a more detailed explanation you can check out that >>> http://www.powerelectricalblog.com/2007/03/armature-and-its-windings.html

It's complicated.

Jim:

I am going to give you a veery simplified answer. First off, ALL adjustable 3rd brush generators will give you more output if you move the third brush in the same direction that the armature rotates when on the car. This is true whether the generator is polarized for positive ground or negative ground. It is also true regardless of where the generator is mounted on the motor and assumes only that the generator is mounted such that it is rotating in its correct designed direction. So the only "questionable" things are home made mountings that might have it rotating backwards. Thus if the device holding the 3rd brush in place slips, the generator rotation will cause the generator to "self advance" the charge rate and smoke shows up. This self advancing characteristic is true for ALL 3rd brush type generators. Ford, Buick, Dodge, Chev, Hudson, Star..... ALL of them do the same self advance and all give more charge when the brush is moved in the direction the armature normally spins.

If you think of the adjustable third brush as being like a tapped transformer you will get more power at the tap (3rd brush) as you move the tap on the transformer up to a higher point on the winding. But there is a positive feedback aspect of this which can cause generator "runaway" condition that burns it up. This is because if the load goes away then the output voltage rises which increases the voltage on the transformer AND at the tap which then increases field current. Increasing the field current increases the output and the whole process builds and builds with higher output producing higher field current producing still higher output. While this actually is all happening magnetically it really doesn't need to be understood that way. if you ground the output then there is no output at the 3rd brush either and the generator will simply spin on its bearings and nothing gets hurt.