[AT] Ford 8n Under Dash Resistor?

[Mike Sloane]

Well, there is another "fly in the ointment": resistance in a wire is 
also dependent on temperature. If you don't use a heavy enough wire, it 
will heat up, and the resistance (and the voltage drop) will change. But 
heavy wires have less resistance than thin ones. Most "ballast" 
resistors are cast into blocks of "sand" to help stabilize the heat. 
Here is a lab worksheet addressing the issue: 
<http://spiff.rit.edu/classes/phys273/manual/temp_coeff.html>.

So, while Steve's idea would work some of the time, it won't work all of 
the time. When I install a ballast resistor in a tractor that I am 
converting from 6 volts to 12 volts, I try to place it in the air stream 
of the fan in an attempt to keep the temperature change to a minimum.

In fact, I have a Farmall 340 that I converted (and installed a 
Pertronix ignition module) not too long ago, and I am having trouble 
getting it started due to poor spark. Once started, it runs fine, and I 
am beginning to suspect that the culprit might be the ballast resistor I 
installed when I did the conversion.

Mike

Dudley Rupert wrote:
> The wire is a good idea, Steve.  If the coil resistance were measured then a
> piece of wire with a resistance roughly 150% of that of the coil would be a
> good starting point.  The objective here is to get a voltage drop across the
> wire of 5 volts.  If the drop is greater than 5 volts then shorten the wire
> and if it is less than 5 then lengthen it.  If a guy had a low ohm wire
> wound potentiometer (variable resister) that might be fun to try too -
> 
> Dudley
> Snohomish, Washington
> 
> -----Original Message-----
> From: at-bounces at lists.antique-tractor.com
> [mailto:at-bounces at lists.antique-tractor.com]On Behalf Of Stephen Offiler
> Sent: Wednesday, January 07, 2009 5:14 AM
> To: Antique tractor email discussion group
> Subject: Re: [AT] Ford 8n Under Dash Resistor?
> 
> 3 volt coil... that's news to me, too.  Thanks Mike.
> 
> If originality is not a goal, but reliability is, you can sometimes
> replicate the function of a wirewound resistor with a coil of regular
> wire.  Depending on the resistance value needed, it might or might not
> be practical.  Of course, the gage and length of the wire are two
> critical variables and some math needs to be done to get it right.
> 
> Steve O.
>