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The four Cluster satellites each carry two instrumentsdesigned for measuring the electric field: a double-probeinstrument (EFW) and an electron drift instrument (EDI).We compare data from the two instruments in a representativesample of plasma regions. The complementarymerits and weaknesses of the two techniques are illustrated.EDI operations are confined to regions ofmagnetic fields above 30 nT and where wave activity and keV electron fluxes are not too high, while EFW canprovide data everywhere, and can go far higher in samplingfrequency than EDI. On the other hand, the EDI technique isimmune to variations in the low energy plasma, while EFWsometimes detects significant nongeophysical electric fields,particularlyin regions with drifting plasma, with ion energy (in eV) belowthe spacecraft potential (in volts).We show that the polar cap is a particularly intricateregion for the double-probe technique, where large nongeophysicalfields regularly contaminate EFW measurments of the DC electric field.We present a model explaining this in terms of enhanced coldplasma wake effects appearing when the ion flow energyis higher than the thermal energy but below the spacecraftpotential multiplied by the ion charge. We suggest that these conditions,which are typical of the polar wind and occur sporadically inother regions containing a significant low energy ion population,cause a large cold plasma wake behindthe spacecraft, resulting in spurious electric fields inEFW data. This interpretation is supported by an analysis ofthe direction of the spurious electric field, and by showingthat use of active potential control alleviates the situation

Electric field measurements on Cluster: comparing the double-probe and electron drift techniques