The maximum charge that can be given to a metallic sphere can be determined using the concept of dielectric strength and the electric field inside the sphere.

Dielectric Strength of Air
The dielectric strength of a material is a measure of its ability to withstand an electric field without breaking down. In the case of air, the dielectric strength is given as 3×10^6 V/m. This means that air can withstand an electric field of up to this value before it breaks down and becomes conductive.

Electric Field inside a Metallic Sphere
When a metallic sphere is charged, the charge distributes itself uniformly on the surface due to the repulsion between charges. The electric field inside a conductor in electrostatic equilibrium is always zero. Therefore, the electric field inside the metallic sphere is zero.

Determination of Maximum Charge
To determine the maximum charge that can be given to the metallic sphere, we need to consider the relationship between electric field, charge, and radius of the sphere.

The electric field inside the metallic sphere is given by the equation:
E = Q / (4πε₀r²)

where E is the electric field, Q is the charge, ε₀ is the permittivity of free space, and r is the radius of the sphere.

Since the electric field inside the sphere is zero, we can set the equation equal to zero:
0 = Q / (4πε₀r²)

Simplifying the equation, we find:
Q = 0

This implies that the maximum charge that can be given to the metallic sphere is zero because the electric field inside the sphere is zero.

Conclusion
Based on the concept of dielectric strength and the electric field inside a metallic sphere, the maximum charge that can be given to a metallic sphere of diameter 6m is zero. This is because the electric field inside the sphere is zero.