in a method for determining charge and / or size of an object suspended in a fluid , the object is introduced , together with the fluid , into an electrostatic trap defining an electrostatic confining potential . the thermal motion of the object in the fluid is observed under influence of the confining potential , and charge and / or size are determined from the observed thermal motion . in particular , the viscous drag on the object yields a measure of its size , while the stiffness of its confinement can be compared with a potential model to reveal the total charge it carries . also disclosed are an apparatus and software for carrying out the method .