Some students are trying to determine whether the radius of a metal sphere affects the amount of charge that it can hold. The students have metal spheres of different radii and known masses that can be attached to a stand by a string, as shown above. The stand is midway between a set of large plates that are connected to a battery to create a nearly uniform electric field in the region between them and near their centers. The students rub a rod with cloth and touch it to each sphere to charge the sphere, and they observe the deflection of the sphere as it is attracted to one plate. They keep adding charge to each sphere until they cannot observe any change in deflection. Which of the following data will give them information that is useful for quantitatively comparing the amount of charge on each sphere?
The number of times they rub the cloth back and forth on the rod
The number of times they touch the rod to the sphere
The string's angle of deflection