Page:EB1911 - Volume 26.djvu/549

Rh idea. It consists in punching, by means of “a puncher,” a series of holes in a strip of paper in such a way that, when the strip is sent through another instrument, called the “transmitter,” the holes cause the circuit to be closed at the proper times and for the proper proportionate intervals for the message to be correctly printed by the receiving instrument or recorder. The most successful apparatus of this kind is that devised by Wheatstone; others were devised by Siemens and Halske, Garnier, Humaston, Siemens, and Little.

In the Wheatstone automatic apparatus three levers are placed side by side, each acting on a set of small punches and

on mechanism for feeding the paper forward a step after each operation of the levers. The punches are arranged as shown in fig. 28, and the levers are adjusted so that the left-hand one moves a, b, c and punches a row of holes across the paper (group 1 in the figure), the middle one moves b only and punches a centre hole (2 in the figure), while the right-hand one moves a, b, d, e and punches four holes (3 and 4 in the figure). The whole of this operation represents a dot and a dash or the letter “a.” The side rows of holes only are used for transmitting the message, the centre row being required for feeding forward the paper in the transmitter. The perforation of the paper when done by hand is usually performed by means of small mallets, but at the central telegraph office in London, and at other large offices, the keys are only used for opening air-valves, the actual punching being done by pneumatic pressure. In this way several thicknesses of paper can be perforated at the same time, which is a great convenience for press work, since copies of the same message have often to be transmitted to several newspapers at the same time.

Although it is quite possible to obtain good signals at a rate corresponding to 600 letters per minute, in practice it is found that such a high speed is not advisable, as it is difficult or impossible for even the most skilled operators properly to handle and transcribe from the “slip” on which the signals are recorded.