The automatic curb sender was a kind of

telegraph key A telegraph key is a specialized electrical switch used by a trained operator to transmit text messages in Morse code in a telegraphy system. Keys are used in all forms of electrical telegraph systems, including landline (also called wire) ...

, invented by

William Thomson, 1st Baron Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, Mathematical physics, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy (Glasgow), Professor of Natural Philoso ...

for sending messages on a submarine communications cable, as the well-known Wheatstone transmitter sends them on a land line. In both instruments, the

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

s are sent by means of a perforated ribbon of

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distrib ...

but the cable sender was the more complicated, because the cable signals are formed by both positive and negative

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

s, and not merely by a single current, whether positive or negative. Moreover, to curb the prolongation of the signals due to electromagnetic induction, each signal was made by two opposite currents in succession: a positive followed by a negative, or a negative followed by a positive. The ''aftercurrent'' had the effect of "curbing" its precursor. For some time, it was the only instrument delicate enough to receive the signals transmitted through a long cable. This self-acting cable key was brought out in 1876, and tried on the lines of the

Eastern Telegraph Company Cable & Wireless plc was a British telecommunications company. In the mid-1980s, it became the first company in the UK to offer an alternative telephone service to British Telecom (via subsidiary Mercury Communications). The company later off ...

References

{{comm-stub Telegraphy