Shown below are the essentials of a commercial mass spectrometer. This device is used to measure the composition of gas samples, by measuring the abundance of species of different masses. An ion of mass $m$ and charge $q=+e$ is produced in source $S$, a chamber in which a gas accelerated by a potential difference $\mathrm{\Delta }V>0$, and then enters a selector chamber, $S_1$, in which there is an adjustable magnetic field ${\overrightarrow{B}}_1$, pointing out of the page and a defecting electric field ${\overrightarrow{E}}_1$, pointing from positive to negative plate. Only particles of a uniform velocity ${\overrightarrow{v}}_1$ leave the selector.

The emerging particles at $S_2$, enter a second magnetic field $\overrightarrow{B_2}$, also pointing out of the page. The particle then moves in a semicircle, striking an electronic sensor at a distance $x$ from the entry slit.