Uncoated tobacco mosaic virus (TMV) can reliably be imaged in humid air (relative humidity above 30%) at relatively high voltages (typically 7 to 10 V) and at very low currents (typically 0.1 to 0.5 pA). Imaging works best with tips that show a small dependence of working distance on tunneling voltage. With such tips, a thickness of the virus of 10 nm and more is measured. When using tips of unsuitable characteristics, heights much smaller than 10 nm and even negative values are found. From these observations, a voltage drop over the TMV on the order of volts can be deduced, which depends on tunneling current and humidity.