Telecom Handouts

I’ve created several handouts to support the Telecommunications course I teach at MSOE.  The links are as follows:  1) Timeline, 2) GrohmannDisplay describes installing the first trans-Atlantic telegraph cable, see: here for more info. 3) DidBellStealGraysIdea for the telephone!

 

Telegraphs

5needleIn 1837, Charles Wheatstone and William Cooke exploited the relatively new discovery of electro-magnetism by Michael Faraday to produce a “direct-read” telegraph (at left). Faraday had demonstrated that an electrical current flowing through a coil of wire generates a magnetic field. Similarly a magnet moving within a coil of wire generates electricity. Wheatstone and Cooke used these ideas to devise a system whereby electricity sent down a wire passes through a coil at the far end. The magnetic field generated in the coil then moves a magnetized compass needle. If the flow of electricity is reversed then so too the magnetic field in the coil is reversed and hence, the compass needle is deflected in the opposite direction. In this way, the direction of electric current flow can be made to move a needle to the left or right. Using this basic principle, Wheatstone and Cooke constructed an instrument that comprised 5 needles arranged on a grid of letters.

5needleFMoving two needles at the same time points to a letter on the grid. Here moving the second needle to the right and the fourth needle to the left points to the letter ‘F’ (see at left). This is known as a direct reading telegraph in the sense that the output shows the letter being sent. In contrast, later telegraph systems (such as Samuel Morse’s telegraph) would use codes to represent letters; and unless you knew the code, then you could not determine what letter was being sent.

On 25th July 1837 with Cooke at Camden Town and Wheatstone at Euston, two stations on the London and Birmingham Railway about nineteen miles apart, messages were exchanged using their five needle telegraph. This event clearly demonstrated that information could be sent over long distance using electricity. There were problems with the five needle telegraph. Firstly, on close inspection it can be seen that only twenty letters can be represented; C, J, Q, U, X and Z couldn’t be sent. Secondly, moving five needles requires six electrical conductors and in 1837 that would be both practically difficult and expensive.

4needleA small improvement was made with the four needle telegraph (at left). Reducing the needles from five to four reduces the number of conductors required but still can only provide for twenty letters to be sent. The reduction of one needle changed the operation of the telegraph in that some letters were sent by moving only one needle. For example H and I, K and L, M and N and O and P were all indicated by the movement of just one needle either left or right. Shown at left is a working model of a four needle telegraph.

The electrical telegraph within the UK was first formally adopted in 1839 by the Great Western Railway. A 13 mile section of telegraph was installed between Paddington and West Drayton at a cost of £3,270.