We report experiments performed on iron particles with radii in the range 40ndash;100 Aring;, produced by pulsed evaporation from an electroplated tungsten filament in a cryogenic helium gas atmosphere. Flight times in electric fields of singly charged particles produced in the evaporation process yield values for gas velocity and charged particle mobility. Particle size distributions are determined both by electron microscopy and from the mobility. The latter method yields particle size distributions quickly and with good statistics, but with indicated radii larger by about 20 Aring; than those determined by electron microscopy. The discrepancy, which is most important for the smallest particles, is attributed to the effect of van der Waals interaction between the iron particles and the gas atoms. Comparison is made with theory, which connects the van der Waals interaction with the dielectric response functions. For particles in this size range, a van der Waals interaction determined frombulkiron data gives a satisfactory explanation of the discrepancy.
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