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Galvanometer, an external instrument measuring current's sustainability for use in telegraph systems. It allows for the detection and measurement of electric currents, especially small amount of electric currents as was the case of telegraph communications. The galvanometer of French Ducreter & E. Roger was part of a telegraph sounder, manufactured especially for use by the military services. The use of galvanometers was essential, especially for submarine wired telegraphy as the electric current would travel many miles without any intermediate signal amplifiers, and would therefore arrive weak to the receiver. Special mirror galvanometers detected said weak signal.