Oscilloscope ground leads

ATTENTION With the next steps you enter Into, this is where line potential occurs. REMOVE the grounding lead from oscilloscope probe to prevent a short circuit from a mains phase via oscilloscope earth ground. 

The bridge should be operated with as low a power level and for as short a time as possible to prevent polarization effects.f If necessary, a preamplifier can be used to increase the imbalance signal before it is applied to the vertical input terminals of the oscilloscope. Be careful that the output leads of the preamplifier are properly connected to the input terminals of the scope, one of which is usually grounded. In order to optimize the accuracy of the measurements, choose Rj values whenever possible so that the variable terminal D is near the center of the slide wire, i.e., the reading Xis near 500 and A3/A4 — 1). 

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