Circuit diagram illustrating a ground loop.  Two circuits share a common ground wire.  The voltage of the ground connection, VG, should ideally be zero.  However if the ground conductor has significant resistance, RG, the current flowing through it from circuit 1 will cause a voltage drop across it VG = I1RG.  Since they share the ground wire, this voltage appears as unwanted interference in the second circuit:  Vout = V2 + VG.
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Chetvorno