Tunnel-effect sensors
Non-contact measurement

It's possible to pass a current between two conductive parts separated by an insulator by applying a potential difference V, despite the potential barrier, through a quantum effect known as the tunnel effect. The current I is a function of the distance d between the electron clouds surrounding the atoms and the level of the potential barrier to be overcome (ϕ ₁ + ϕ ₂ )/2 (" 3 eV) according to the relationship (figure 19 ): $I\propto e^{\left[-A{\left(\frac{{\phi }_1+{\phi }_2}{2}\right)}^{\text{1/2}}d\right]}$