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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