Just like the "total spin angular momentum" cannot be measured, neither can the "total spin magnetic moment" be measured. Equations (), (), () give the physical observable, that component of the magnetic moment measured along an axis, relative to or along the applied field direction. Assuming a Cartesian coordinate system, conventionally, the "z"-axis is chosen but the observable values of the component of spin angular momentum along all three axes are each ±. However, in order to obtain the magnitude of the total spin angular momentum, be replaced by its eigenvalue, where "s" is the spin quantum number. In turn, calculation of the magnitude of the total spin magnetic moment requires that () be replaced by: