After his experience on the 1857 voyage, Thomson realised that a better method of detecting the telegraph signal was required. He spent the time waiting for a new voyage inventing his mirror galvanometer. This was an extremely sensitive instrument, much better than any existing equipment. He requested £2,000 from the board to build several of them, but was only given £500 for a prototype, and permission to trial it on the next voyage. The mirror galvanometer was extremely good at detecting the positive, or negative edge of a telegraph pulse respectively representing a Morse 'dash' and a Morse 'dot' (this was the standard system on submarine cables – both pulse types were the same length, unlike overland telegraph). Thomson believed he could use this instrument with the low voltages from regular telegraph equipment even over the vast length of the Atlantic cable. He successfully tested the instrument on 2,700 miles of cable in underwater storage at Plymouth.