The potential distribution in front of a Ag ring electrode during the reduction of peroxodisulfate was measured with potential microprobes. Inhomogeneous stationary potential distributions were observed when using a Haber-Luggin capillary, i.e., placing the reference electrode close to the working electrode on the axis of the ring. It is shown that such an arrangement introduces a negative global coupling into the system which destabilizes homogeneous stationary states if the current-potential characteristics exhibits a negative differential resistance. Further consequences of the negative global coupling are discussed, and the effect of an additional, external series resistor is demonstrated.