Mirror galvanometer

[1] This instrument was far more sensitive than any which preceded it, enabling the detection of the slightest defect in the core of a cable during its manufacture and submersion.

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.

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.

[1] The following is adapted from a contemporary account of Thomson's instrument:[2] The mirror galvanometer consists of a long fine coil of silk-covered copper wire.

In the heart of that coil, within a little air-chamber, a small round mirror is hung by a single fibre of floss silk, with four tiny magnets cemented to its back.

The practical advantage of this extreme delicacy is that the signal waves of the current may follow each other so closely as almost entirely to coalesce, leaving only a very slight rise and fall of their crests, like ripples on the surface of a flowing stream, and yet the light spot will respond to each.

What with this shifting of the zero and the very slight rise and fall in the current produced by rapid signalling, the ordinary land line instruments are quite unserviceable for work upon long cables.Moving coil galvanometer was developed independently by Marcel Deprez and Jacques-Arsène d'Arsonval about 1880.

Another important feature is self-damping generated by the electro-magnetic forces due to the currents induced in the coil by its movements the magnetic field.