The Cooke and Wheatstone telegraph consisted of a number of magnetic needles which could be made to turn a short distance either clockwise or anti-clockwise by electromagnetic induction from an energising winding. The direction of movement was determined by the direction of the current in the telegraph wires. The board was marked with a diamond shaped grid with a letter at each grid intersection, and so arranged that when two needles were energised they would point to a specific letter.
The number of wires required by the Cooke and Wheatstone system is equal to the number of needles used. The number of needles determines the number of characters that can be encoded. Cooke and Wheatstone's patent recommends five needles, and this was the number on their early demonstration models. The number of codes that can be obtained from 2, 3, 4, 5, 6 ... needles is 2, 6, 12, 20, 30 ... respectively.
At the sending end there were two rows of buttons, one button for each coil in each row. The operator selected one button from each row. This connected two of the coils to the positive and negative ends respectively of the battery. The other end of the coils was connected to the telegraph wires and the coils at the receiving station. The other end of the receiving coils, while in receive mode, were all commoned together. Thus the current flowed through the same two coils at both ends and energised the same two needles. With this system the needles were always energised in pairs and always rotated in opposite directions.
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