When a message is sent over the wires, the actual time of making the signals is more than is required for the current to pass from place to place. This fact has been utilised by the inventors of methods whereby two or more messages may not only be sent the _same_ way along the same wire, but may also be sent in _different_ directions. Messages are “duplex” when they travel across one another, “multiplex” when they travel together. The principle whereby several instruments are able to use the same wire is that of _distributing_ among the instruments the time during which they are in contact with the line. Let us suppose that four transmitters are sending messages simultaneously from London to Edinburgh. Wires from all four instruments are led into a circular contact-maker, divided into some hundreds of insulated segments connected in rotation with the four transmitters. Thus instrument A will be joined to segments 1, 5, 9, 13; instrument B to segments 2, 6, 10, 14; instrument C with segments 3, 7, 11, 15; and so on. Along the top of the segments an arm, connected with the telegraph line to Edinburgh, revolves at a uniform rate. For about 1/500 of a second it unites a segment with an instrument. If there are 150 segments on the “distributor,” and the arm revolves three times a second, each instrument will be put into contact with the line rather oftener than 110 times per second. And if the top speed of fifty words a minute is being worked to, each of the fifteen signals occurring in each second will be on the average divided among seven moments of contact. A similar apparatus at Edinburgh receives the messages. It is evident that for the system to work satisfactorily, or even to escape dire confusion, the revolving arms must run at a level speed in perfect unison with one another. When the London arm is over segment 1, the Edinburgh arm must cover the same number. The greatest difficulty in multiplex telegraphy has been to adjust the timing exactly. Paul la Cour of Copenhagen invented for driving the arms a device called the Phonic Wheel, as its action was regulated by the vibrations of a tuning-fork. The wheel, made of soft iron, and toothed on its circumference, revolves at a short distance from the pole of a magnet. As often as a current enters the magnet the latter attracts the nearest tooth of the wheel; and if a regular series of currents pass through it the motion of the wheel will be uniform. M. la Cour produced the regularity of current impulses in the motor magnet by means of a tuning-fork, which is unable to vibrate more than a certain number of times a second, and at each vibration closed a circuit sending current into the magnet. To get two tuning-forks of the same note is an easy matter; and consequently a uniformity of rotation at both London and Edinburgh stations may be insured.