Polarized 'guillotine' relay, designed by W Andrews, England, 1862-1866.
A relay is an electrically operated switch, and allowed weak telegraph signals to be boosted by a local power source. Polarised electrical relays work by using an armature in between the poles of a permanent magnet in order to increase sensitivity. This one has an armature in the form of a semicircular magnet, placed horizontally and vibrating between two electromagnets. The current passes through both sets of coils, which are wound so that one attracts, and the other repels the armature.