Thomson decided that he needed an extremely sensitive instrument after he took part in the failed attempt to lay a transatlantic telegraph cable in 1857. He worked on the device while waiting for a new expedition the following year. He first looked at improving a galvanometer used by Hermann von Helmholtz to measure the speed of nerve signals in 1849. Helmholtz's galvanometer had a mirror fixed to the moving needle, which was used to project a beam of light onto the opposite wall, thus greatly amplifying the signal. Thomson intended to make this more sensitive by reducing the mass of the moving parts, but in a flash of inspiration while watching the light reflected from his monocle suspended around his neck, he realised that he could dispense with the needle and its mounting altogether. He instead used a small piece of mirrored glass with a small piece of magnetised steel glued on the back. This was suspended by a thread in the magnetic field of the fixed sensing coil. In a hurry to try the idea, Thomson first used a hair from his dog, but later used a silk thread from the dress of his niece Agnes.