The device presented here is a model of the electromagnetic relay invented around 1850 by the Swiss watchmaker Mathäus Hipp (1813-1893), which took the place of those previously used in the entire Swiss telephone network and which allowed a reduction in the number of relays from 23 to 11.
The need to use an automatic repeater in telegraphy derived from the fact that when the electricity that carried a message arrived at the telegraph receiver at the end of a long line it was sometimes so weak that it would fail to make it work. In such cases, it was of little use to increase the power of the generator supplying the line. The discovery of the electromagnetic relay allowed Charles Wheatstone (1801-1875) in 1858 to solve this problem in a quite simple way by introducing a device that opened and closed an electrical circuit under the action of a weak current in arrival. This circuit, powered by a local generator, was capable of supplying a sufficiently intense current to allow the receiver to work (the typical function of a repeater relay).
An electromagnetic relay is essentially made up of an electromagnet whose mobile armature is fixed to one of the arms of a lever. When electricity circulates through the electromagnet, the armature is attracted by a circuit breaker which has contacts fixed at the extremity of the other arm of the lever and closes another circuit. A spring keeps the lever in the rest position with the circuit breaker open when no current is circulating through the electromagnet. It is clear that the relay is particularly suited for the transmission of messages in Morse code; indeed, in the second circuit, electricity circulates so long as a current is circulating through the electromagnet and if the electromagnet is connected to a telegraph line, the relay, in concomitance with a dash of a dispatch will transmit a dash to the receiver and in concomitance with a dot will transmit a dot. If, with the help of a relay on the spot, the problem caused by a weak signal in arrival could not be solved, it was necessary to divide the line into two or more trunks; also in this case it was necessary to use a relay that would automatically transmit dispatches from one intermediate station to the next and then to the receiver.
Hipp's relay differs from other relays in use at that time in that the lever with the mobile armature was equipped with a system of two spiral springs under different tensions pulling in different directions.
This setup, in which it is the difference in tension of the two springs that brings the armature into the rest position, greatly decreases the inertia of the vibrating rod, thus making its movements quicker.
The relay on display here has a square wooden base (14 x 14 cm), an overall height of 20 cm and is protected by a glass bell.
The electromagnet is composed of two vertical, cylindrical coils wound around two arms of a U-shaped soft iron nucleus.
The mobile armature is set crosswise at the extremity of a lever that oscillates between two blades with an amplitude that can be regulated by means of two screws.
The two opposing springs are inside a hollow brass column: the extremity of one spring is attached to the top of the column and the other to the base of the column; they act on the lever by means of a small rectangular ring to which they are fixed with the other extremity.
A brass screw passing through the extremity of the column allows regulation of tension on the upper spring and a pin which can run along a vertical slit in the column acts as an index. A small lever, placed at the front of the device, allows regulation of tension on the lower spring and a movable needle indicates its degree on a semicircular scale.
Two horizontal clamping screws, fixed to the edge of the wooden base, communicate with the terminals of the electromagnet and are connected to the telegraph line for reception of dispatches.
Two other clamping screws, one of which in communication with the contact screw which is insulated from the column, and the other with the lever of the mobile armature, are for the purpose of retransmitting the dispatch to the receiver connected in series to the local generator.
Daguin (1878), T III, p. 825
De La Rive (1858), T. III, pp. 395 e 408
Du Moncel (1874), T III, p. 406