In reply to a message from craig_g sent Fri 22 Apr 2016:
Hi Craig, not dumb questions at all! If I knew the correct
voltage for every case, for every degaussing coil, to reduce
the magnetism by the exact amount to get to a specific
target MPH, I would open an instrument repair shop. Every
setup will be different and will likely require different
degree of degausing, hence the need to vary the level of
degaussing by varying the AC voltage to it with a variac.
I have not tried this but another possible way to apply a
varying level of degaussing is to mount the speedometer on
it’s cable approx 12’’ above the degaussing coil. Start the
drill, apply the fixed 30 to 60 volts AC to the degaussing
coil. Then, with the coil energized, slowly raise the coil
toward the bottom of the speedometer where the magnet is
spinning and watch the MPH needle display. When you reach
your target MPH, switch off the voltage or quickly lower the
coil. If you do this, just be sure to insulate yourself
from the coil and whatever you do, isolate the coil (and
yourself) electrically from the 120 V AC power line with a
step down transformer. Safety first, because you will want
to be around to enjoy your new calibrated speedometer!
If you try it, let me know. And remember, if you over
degauss, re-magnetize and apply less degaussing. I tried
mine over and over again and repeated the test at least 5 times.
The last test I did was to magnetize my speedometer magnet
to meet an 1120 TPM. The reason I went for 1120 TPM is that
I am converting my diff from a 3.54 to a 3.07. With a 3.07
diff, the display should read 60 MPH at 1120 TPM (or RPM) So
after magnetizing, spinning up the speedometer with my 1400
RPM drill, it was reading 76 MPH. The target for the 1120 is
60*1400/1120 = 75 MPH So, my target MPH was 75 and the
speedometer was reading 76 MPH, close enough for me. I left
it alone and did not bother to degausse. I was lucky. Maybe
I’ll loose that 1 MPH in 5 or so years. Who knows?
So the inverse to your question of how much degaussing does
it take to lower it to your target MPH is, how much
magnetizing does it need to raise it to your target MPH.
I hope this helps explain it.–
The original message included these comments:
I’m guessing the voltage can’t be determined and/or the
degaussing effect is not linear. But I’m not opposed to
asking dumb questions!
–
Anthony, Toronto Ontario, 69 OTS
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