Polarised Telegraph Relay
A tongue or armature is pivoted at one end so as to swing between two iron blocks. These blocks are set on the iron cores of an electro-magnet. A steel permanent magnet inside the case is so placed that it magnetises both the blocks South, and the free end of the tongue North. When a current passes through the coils of the electro-magnet it strengthens the magnetism of one block and diminishes that of the other: consequently the tongue is drawn to one side and makes a battery contact on the stop opposite to the spring. The other stop is insulated. This battery contact closes the circuit of another electro-magnet called a 'Sounder' and messages are read by sound from this according to the motions of a Key controlling the current which passes through the relay coils. The spring has been added to ensure the opening of the tongue in the absence of current. When the tongue is placed midway, and no current passes, the tongue is equally attracted by both blocks.
The relay may be used either with currents of one direction or with reversals of current.