George, I too have an old Simpson 260. The last time I tried to use it, it wouldn't work. I suppose the switch contacts are corroded from lack of use.
The reason the old analog units had a zero adjustment wasn't so much to allow for the lead resistance as it was to compensate for the battery condition. They don't have a real stable current source (I think it's just a resistor), so the zero would be all over the map.
Something to be aware of with digital units is that the "continuity" or "diode check" scale is NOT a resistance measurement! It actually displays VOLTS, not OHMS. The reason for this is that a good diode has a forward bias voltage of around 0.7 volts. A lot of folks use the diode check range, see a reading of less than 1.0 and assume the resistance is zero. NOT SO! When in doubt, double-check the actual resistance.
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Today's Featured Article - Ford Part Number Trivia - by Forum Participants. "Replaced by" means the part was superseded. All of my part books date back to 1964 and New Holland have changed some part numbers. They usually put the old Ford part number on the package. I was suppressed when I looked up the part number of the auxiliary drive shaft because for some reason the part number went through a radical change and it lost its "Basic Part Number". Ford part numbers follow the following rules. Most part numbers are in three parts. The middle part is called the
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