Some electronics applications need a switch that activates without any physical contact. A reed switch with contacts encased in a hermetically glass tube activates when it comes near a magnetic field. A typical application would be to have an alarm on a door, with the switch mounted just above the door jamb and the magnet attached near the top of the door. When the door closes, the switch activates. Reed switches can open or close a connection when activated.
Set the multimeter to read ohms by turning the dial to an ohms position. Touch the leads together to test it. You should read zero ohms on the meter scale.
Connect the ohmmeter to the two leads on the reed switch. Note the reading. Do not disconnect the meter. You should read either infinity for open or zero ohms for closed.
Place the magnet near the reed switch. The switch should operate. If the meter read infinite before, it should now read zero, and vice versa.
Some reed switch kits come with a magnet for open door alarms.