Air breaks down and conducts (sparks) when the electric field in it reaches about $3 \times 10^6\ \text{V/m}$. A metal sphere of radius $10\ \text{cm}$ stands in air, far from other objects. Take $\dfrac{1}{4\pi\varepsilon_0} = 9 \times 10^9\ \text{N m}^2\,\text{C}^{-2}$. The largest charge it can hold, and its potential then, are