# Magnetic Field Strength and a Spherical Superconductor

While reading through notes of a class I am taking, I came across an example exercise about a superconducting sphere in an applied magnetic field. In this example it states the following:

$$H_{in}=H_{0}-\frac{1}{3}M$$

Where $H_{0}$ is the applied magnetic field strength, $H_{in}$ is the magnetic field strength inside the sphere and M is magnetisation. In the notes for this example, it explains this is a result of diamagnetism but does not explain the origin of this relationship any further. How would one arrive at such an equation?