I would like to know the maximum voltage that a metal sphere can hold from applying negative charge to it. My thoughts are that the maximum voltage is the breakdown voltage of the surrounding medium (air in this case). I assume that at that voltage, electrons would arc from the sphere to the nearest object. I found that air actually has a breakdown electric field strength of 3 MV/m, which I assume to mean that if there is an object one meter away from a sphere charged to 3 MV, breakdown will occur (likewise, if there is an object three meters away, I can go up to 9 MV before breakdown occurs, etc.). Since the work required to apply another charge increases with each charge applied, I imagine that perhaps special equipment would be required to actually achieve such a large voltage. What equipment is capable of this? I would also like to know the best practical (household or within a reasonable purchase) way of applying a specific amount of negative charge to a metal sphere. I imagine that I could charge a capacitor and discharge half of the voltage into the sphere since the system would achieve equilibrium once the voltage of the capacitor matches the voltage of the sphere. I imagine a coulombmeter would otherwise be of great service.