In article , Harold and Susan Vordos says...

... Remember, you are not really measuring with a DTI
under setup conditions, but looking for anomalies so they can be eliminated
or minimized. The indicator, in use, behaves as a comparator. It's not
really "measuring", although it does display in increments that translate
into measurements.

The analog in the world of electrical measurements is a "null detector"

The actual linear sensitivity of a last word type indicator varies
a great deal across its range because of the cosine error between the
angle of the point and the surface it's sweeping. But it will be
most sensitive at one spot and if the needle nulls (never moves)
during the measurement, one can be sure the feature and the machine
axis are lined up. The non-linearity of the detector drops out.

Jim


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