It is irrelevant what potential the resistor ends are with respect to circuit ground. The point at which the current enters the resistor must be connected to the arduino ground and the end at which the current flows out goes to the arduino sense input.Note : If the potential of either end of the resistor is relatively high with respect to true ground then you are working on a potentially hazardous system and you should not consider direct connection to the arduino. If this is the case then you require a galvanic isolation module.jack
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