can be seen in a general way without detailed analysis. For the determination of the radius of the drop is equivalent to finding its weight, since its density is known. That we can find the charge on the drop as soon as we can determine its weight is clear from the simple consideration that the velocity under gravity is proportional to its weight, while the velocity in a given electrical field is proportional to the charge which it carries. Since we measure these two velocities directly, we can obtain either the weight, if we know the charge, or the charge, if we know the weight. (See equation 9, [p. 70].)
V. WEIGHING THE DROPLET
The way which was first used for finding the weight of the drop was simply to solve Stokes’s uncorrected equation (11) ([p. 91]) for a in the case of each drop. Since the curve of [Fig. 4] shows that the departures from Stokes’s Law are small except for the extremely slow drops, and since
appears in the second power in (11), it is clear that, if we leave out of consideration the very slowest drops, (11) must give us very nearly the correct values of
. We can then find the approximate value of
by the method of the next section, and after it is found we can solve (15) for the correct value of