Firstly I don't think its limited to "early American trucks" as all the U=
K
vehicles virtually up to the introduction of alternators were positive
ground too.
Unfortunately electricity doesn't flow from positive to negative as is
convenient for the brain to handle. It is the flow of electrons,
negatively charged, from negative to positive, much more awkward to
envisage.
As a further complication the movement of a negative electron creates a
positive "hole"! This is best imagined as a line of pegs on a solitaire
board as you move one peg at a time into a spare hole the pegs move one w=
ay
(electrons) and the hole appears to move the other.
I suspect the very first use of a dynamo (DC generator) on a vehicle
followed the rules of physics: the flow of electricity is by negative
electrons, the battery negative therefore is live supplying electrons
"outwards" to the vehicle circuits. Everybody then followed suit.
There have been various theories that with positive ground the battery
negative terminal corrodes and with negative ground the vehicle itself
corrodes; I've never seen any detailed proof of this.
It is an easy matter to reverse the polarity of a vehicle dynamo and old
vehicles have no polarity dependant original equipment, save an ammeter, =
if
fitted, that will read in reverse unless the wires to it are exchanged.
For a military vehicle with a radio fit it is worth checking if the
equipment has a defined ground polarity, most don't and are fully floatin=
g;
the supply would have to be reversed though.
To flip the polarity of a dynamo momentarily connect it in reverse to a
battery eg., strap the output and field terminals, connect a battery neg =
to
case and pos to these terminals. You will have a positive output generat=
or
now.
Even easier in a vehicle, reverse the battery then push the
electro-magnetic cut-out in, be ready to pry the armature apart again as =
it
will energise and whilst made, takes tens of amps from the battery. This=
flips the residual magnetic field of the generator that begins the
generation process, and defines the polarity, from the rest state.
You CANNOT do this with an alternator.
Richard
(Southampton UK)
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