According to the inductive theory of thunderstorm electrification, a small ice crystal bouncing off the underside of a larger hydrometeor falling in the presence of an electric field should remove some of the field-induced polarization charge from the surface of the larger particle; subsequent gravitational separation of the two particles should then result in a rapid growth of the electric field. However, because of the finite surface conductivity of the ice, it is not obvious that the polarization charges can flow along the ice surface in the short interaction time. We report laboratory experiments on the effect of an external electric field on the charge transferred during individual collisions of small (typically, 100-μm) ice spheres with larger ice targets. When the surface conductivity of the ice was artificially increased by using 10-2 M NaCl, the full theoretical charge transfer was observed, but at 10-5 M NaCl, no field-dependent charge transfer could be detected. Airborne measurements of cloud and precipitation purity have shown that 10-4 M NaCl is occasionally reached in low-level stratiform clouds in polluted regions, but even with this degree of contamination the inductive mechanism operates at less than 20% of its potential efficiency at -10 ¿C; at lower temperatures this fraction is reduced still further. We conclude that the inductive mechanism cannot account for the observed electrification of thunderstorms unless other contaminants are having an unexpectedly large effect on the conductivity.