Measurement and Instrumentation
The spring material used in a spring control device should have the following property.

Should have low specific resistance
Should non magnetic
Most be of low temperature co-efficient
All of the listed here

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Measurement and Instrumentation
In DC potentiometer measurments, a second reading is often taken after reversing the polarities of the DC supply and the unknown voltage and the average of the two reading is taken. This is with a view of eliminate the effects of

  Stray thermal emfs.
 Stray magnetic fields.
 Errorneous standardisation.
 Ripples of the DC supply.

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