Five-needle Telegraph
The five-needle telegraph with twenty possible needle positions was six codes short of being able to encode the complete alphabet. The letters omitted were C, J, Q, U, X and Z. A great selling point of this telegraph was that it was simple to use and required little operator training. There is no code to learn, as the letter being sent was visibly displayed to both the sending and receiving operator.
The Paddington to West Drayton telegraph originally used six wires rather than five, although it was a five-needle system. The sixth wire was to provide a common return so that the needles could be operated independently, thus giving the possibility of more available codes. Using these codes, however, would have required more extensive operator training since the display could not be read on sight from the grid as the simple alphabetic codes were. Telegraph systems were later to use earth return to avoid the need for a return wire, but this principle was not established at the time of Cooke and Wheatstone's telegraph. The economic need to reduce the number of wires in the end proved a stronger incentive than simplicity of use and led Cooke and Wheatstone to develop the two-needle telegraph.
Read more about this topic: Cooke And Wheatstone Telegraph, Operation