where $R_{\rm H}=-\frac{1}{ne}$ is the _Hall coefficient. So by measuring the Hall coefficient, we can obtain $n$, the density of free electrons in a material.

where $R_{\rm H}=-\frac{1}{ne}$ is the *Hall coefficient*. So by measuring the Hall coefficient, we can obtain $n$, the density of free electrons in a material.

While most materials have $R_{\rm H}<0$,interestinglysomematerialsarefoundtohave$R_{\rmH}>0$. This would imply that the charge of the carriers is positive. We will see later (chapter 17) how to interpret this.