It is possible under some circumstances that the accuracy of data on electrical parameters inside thunderclouds obtained by use of instrumented balloons may sometimes be adversely affected by the balloon platform itself. If the balloon rigging becomes conductive, polarization of the balloon system, corona emissions, and electrical generation of charged water particles may act to alter the parameters that are being measured. Laboratory measurements of conduction currents flowing along nylon monofilaments, sometimes used as insulating supports for instruments carried beneath balloons, revealed a large conductance when the filaments were wet or exposed to high humidity and the ambient temperature was above O ¿C. The conductance of a piece of monofilament at high humidity increased by more than 4 orders of magnitude as the air temperature rose from -15¿ to +20 ¿C. The conductance also depended strongly on the monofilament's surface cleanliness. At 20 ¿C in high humidity, currents of up to 0.3 μA flowed along a piece of monofilament in response to a potential gradient of 2000 V/m. Application of large potential gradients to a monofilament continually wetted, as might be expected as a worst-case scenario in clouds or rain, produced corona emissions and electrical generation of charged drops. These results indicate that especially in the warmer, lower regions of clouds, currents may flow in the rigging of balloon-borne instrumentation suffcient to reduce the ambient atmospheric electric fields and to generate a population of artificially charged water particles. In order to ensure credibility of electrical measurements in such situations, it is essential that conduction along the rigging be minimized by the use of hydrophobic materials, and it is desirable that means for monitoring leakage currents be included in the instrumentation. ¿ American Geophysical Union 1990