It is important to first review some of the key terms used by engineers to describe reed relays. The coil is composed of multiple turns of fine insulated copper wire, as shown on this slide, and is used to generate a magnetic field strong enough to close the contacts of a reed switch. When a voltage is applied to the coil, the generated current will produce a magnetic field that will operate, or close, the reed switch contacts, usually positioned within the coil. Removing the voltage from the coil will allow the contacts to open, or deactivate the contacts.