def set_bfield(self, B_G):
        """Set the strength of the local magnetic field.

        **Call signature**

        *B_G*
          The magnetic field strength, in Gauss
        Returns
          *self* for convenience in chaining.
        """
        if not (B_G > 0):
            raise ValueError('must have B_G > 0; got %r' % (B_G,))

        self.in_vals[IN_VAL_B] = B_G
        return self