Fisher returned to "Excellent" in 1864 as a gunnery instructor, where he remained until 1869. Towards the end of his posting he became interested in torpedoes, which were invented during the 1860s, and championed their cause as a relatively simple weapon capable of sinking a battleship. His expertise with torpedoes led to his being invited to Germany in June 1869 for the founding ceremony of a new naval base at Wilhelmshaven, where he met King William I of Prussia (soon to become German emperor), Bismarck and Moltke. Perhaps inspired by the visit, he started preparing a paper on the design, construction and management of electrical torpedoes, the cutting-edge technology of the time.