sphere of 3" radius = 0.0762m = rs

surface of sphere = 4 x pi x rs^2

surface of a tube = L x 2 x pi x rt

4 x pi x rs^2 = L x 2 x pi x rt

2/L x rs^2 = rt = radius of a tube

0.0762^2 x 2 = 0.01161288m = rt

area = pi x rt^2

0.01161288^2 x pi = 4.2367198679004597751047316571645e-4 = area of this tube

inductance of a tube = its area times the permeability of free space

4.2367198679004597751047316571645e-4 times 1.26e-6 = 5.3382670335545793166319618880273e-10 = 533.8267pH