# I The component of the total spin in the direction of H

1. Jan 11, 2019

### Another

i don't understand,
Why The component of the total spin in the direction of H is $M_s = \frac{1}{2}(N_{up}-N_{down})$

2. Jan 11, 2019

### Staff: Mentor

Because $N_{\uparrow}$ particles contribute $+1/2$ and $N_{\downarrow}$ particles contribute $-1/2$ to the total magnetisation.