English: The principle of the relay may be understood by referring to the figure. It will be seen that the ends of the local windings are joined to terminals B and C and their centre to terminal A, which is joined through an adjustable resistance Y to the relay tongue. This resistance is for regulating the local current and keeping it below the value of the steady current through the line coils. Terminal B is connected to earth through a condenser K, while C has a resistance coil X in its earth lead.

Assuming that there is no current in the line coils and that the relay tongue has just reached the marking contact, there will then be a momentary rush of current through the winding AB to charge the condenser K, in a direction to keep the tongue to the marking side, thus preventing any tendency of the tongue to rebound. This charging current dies away rapidly, however, and directly its strength falls below the steady current flowing through the winding AC, the preponderance of the latter causes the tongue to move toward the spacing contact. Immediately the tongue leaves the marking contact, the condenser K discharges through both windings BA and AC in such a direction as to accelerate the movement of the tongue, so that its transit time from one contact to the other is thereby lessened.

When the tongue reaches the opposite contact the condenser K is again charged, but this time from the other pole of the battery; a similar cycle of effects therefore takes place on that side and the tongue moves in the reverse direction. In this manner the relay tongue is kept vibrating, at a speed depending on the values given to the condenser and resistance. In practice the adjustments are such that the rate of vibration of the tongue under the control of the local current is approximately equal to the rate at which the transmitter at the distant station sends reversals at working speed. When this obtains, the signals passing through the line coils merely determine the length of time that the tongue remains in contact with either stop, its movement therefrom being effected by the local current through the local windings as soon as the strength of the line current falls below that of the local current in the coil AC. It is this effect combined with the action of the condenser in lessening the time of transit of the tongue, that enables a higher speed of working to be attained on long and difficult circuits than if ordinary polarized relays were used.