For an undergraduate physics laboratory experiment we often make two changes in Millikan’s procedure. First, we use plastic balls of about 1 micrometer (μm or micron) diameter, for which we can measure the mass easily and accurately. This avoids the measurement of the oil drop’s terminal velocity and the dependence on Stokes’s law. The small plastic balls are sprayed through an atomizer in liquid solution, but the liquid soon evaporates in air. The plastic balls are observed by looking through a microscope. One other improvement is to occasionally bombard the region between the plates with ionizing radiation, such as an electron (beta particle) from a radioactive source. This radiation ionizes the air and makes it easier for the charge on a ball to change. By making many measurements we can determine whether the charges determined from Equation (3.8) are multiples of some basic charge unit.
q=\frac{m g d}{V} (3.8)
