Go back to article: A tale of two telegraphs: Cooke and Wheatstone’s differing visions of electric telegraphy

The needles and the dial

Three months after their first meeting, in February of 1837, Cooke and Wheatstone filed their first joint patent for a telegraph instrument (Cooke and Wheatstone, 1837). This patent describes improvements that made the instrument practical – presumably in reference to Cooke’s earlier mechanical telegraph that was never operational. One of the important contributions from Wheatstone consisted of an apparatus known as the Hatchment Dial with its characteristic diamond-shaped vertical board. This device employed magnetic needles that were set on a horizontal line in the centre of the dial. The needles pointed up at rest, or left or right when deflected by the action of an electric current. Letters were indicated by the simultaneous deflections of two needles in contrary directions. Wheatstone’s innovations were the vertical mounting of these needles, the dial’s alphabetic arrangement, and the so-called ‘permutating’ keyboard that operated the needles.

After a short practice, any literate person could operate the device proficiently as it offered direct read capability. This was not a small instrument, let alone easily transportable, but it already revealed Wheatstone’s inclination towards designing telegraphic instruments for use by the general public.

Figure 1

Colour photograph of Wheatstones Hatchment Dial which is set in wood organized in a diamond shape and marked with twenty letters

Wheatstone’s Hatchment Dial. This specimen is organised in a diamond shape and marked with twenty letters, the letters C, J, Q, U, X and Z being omitted to save the expense of an additional needle and associated wire. In the five-needle model described in Sheet I of the joint patent of 1837, letters were indicated by the deflections of two needles in contrary directions, while numerals (not shown in this specimen) were indicated by the deflection of only one needle, and required the addition of a sixth wire. At rest, the needles were in a neutral (vertical) position.

The Hatchment Dial instrument was successfully tested in 1837 during the London & Birmingham Railway experiment from Camden Town to Euston, but the cost of constructing and laying the five wires was a financial and engineering burden.[6] For the next project, which took place at the Great Western Railway Company, Cooke designed an enhanced version of his mechanical telegraph which incorporated Wheatstone’s electro-magnetic improvements: a two-needle apparatus that operated on only three wires. This instrument was an efficient signalling device in railway operation, but it did not have a direct read capability in alphabetic operation as it used coded sequences of needle deflections to communicate the messages (for instance, two deflections to the right to mean X, one to the right and one to the left to mean Y, etc.). It was to be worked by skilled operators.

Figure 2

Colour photograph of Cooke and Wheatstone needle instrument from 1837 set in polished wood

Cooke & Wheatstone needle instrument, three wires operation, 1837 specification. This instrument, or a similar one, is likely to have been used on the Great Western Railway project.

Wheatstone was also determined to reduce the number of wires, while preserving at the same time the benefit of direct read. In 1840, he designed a new instrument that was partly based on an earlier design by Francis Ronalds.[7] This instrument required two wires to operate and it was built around three components: a transmitter, a receiver, and an alarm. The receiver was designed around a clockwork mechanism. It employed a step-by-step rotating circular dial that moved one step at a time, in one direction, with each electrical impulse received from the remote transmitter. The dial was divided into twenty-four sectors identified by letters, numbers and special characters, and these symbols were presented in a small aperture on the front of the instrument. The transmitter worked by positioning a finger at the edge of a capstan on a space associated with the letter or number to be sent, and rotating the capstan until it reached an index (see Figure 3). This action caused the mechanism behind it to make and break the circuit, sending an electrical impulse as each preceding character passed in front of the index. This was, in effect, an early version of the rotary dial used on future telephones. This instrument was called the escapement or the dial telegraph. Unlike the Hatchment Dial, it was small enough to be carried in a small case and was capable of operating at about thirty characters per minute.

Figure 3

Colour photograph montage showing Wheatstones escapement or dial telegraph showing the transmitter and receiver side by side

Wheatstone’s escapement or dial telegraph (1840 specification) employed two wires for its operation and was based on the step-by-step technology. The transmitter (or communicator) can be seen on the left, the receiver (or indicator) on the right.

The step-by-step technology featured prominently in the joint patent filed in 1840 (Wheatstone and Cooke, 1840).[8] The patent also included improvements made by both Wheatstone and Cooke for the needle instrument that we saw earlier – the one used for the Great Western Railway project.

Component DOI: http://dx.doi.org/10.15180/170804/003