An electromagnetic lock, magnetic lock, or maglock is a locking device that consists of an electromagnet and an armature plate.

The principle behind an electromagnetic lock is the use of electromagnetism to lock a door when energized. The holding force should be collinear with the load, and the lock and armature plate should be face-to-face to achieve optimal operation.

The magnetic lock relies upon some of the basic concepts of electromagnetism. Essentially it consists of an electromagnet attracting a conductor with a force large enough to prevent the door from being opened. In a more detailed examination, the device makes use of the fact that a current through one or more loops of wire (known as a solenoid) produces a magnetic field. This works in free space, but if the solenoid is wrapped around a ferromagnetic core such as soft iron the effect of the field is greatly amplified. This is because the internal magnetic domains of the material align with each other to greatly enhance the magnetic flux density.

Using Ampère's circuital law, it can be shown that the magnetic flux density ${\displaystyle B}$ induced by a solenoid of effective length ${\displaystyle L}$ with a current ${\displaystyle I}$ through ${\displaystyle N}$ loops is given by the equation:

$${\displaystyle B={\frac {\mu _{0}\mu _{r}IN}{L}}}$$

The force ${\displaystyle F}$ between the electromagnet and the armature plate with surface area ${\displaystyle S}$ exposed to the electromagnet is given by the equation:

$${\displaystyle F={\frac {B^{2}S}{2\mu _{0}}}}$$

In both equations, ${\displaystyle \mu _{0}}$ represents the permeability of free space and ${\displaystyle \mu _{r}}$ the relative permeability of the core.