Kelvin's mirror galvanometer with lamp and stand, 1858.
© Science Museum / Science & Society Picture Library
The Irish physicist Sir William Thomson, later Lord Kelvin (1824-1907), invented the mirror galvanometer in 1858. This instrument consists of a mirror attached to a small steel magnet. The mirror is suspended by a thread between two copper coils. When an electric current pases through the coils, the magnet moves according to the magnetic field created. Light pasing from a lamp through a slat in a screen is reflected by the mirror onto a graduated scale. Changes in the electro-magnetic charge would produce movements on the screen. The mirror galvanometer was used in the first Atlantic telegraph transmisions in 1858, and Thomson amased a large personal fortune from cable patents and consultancy fees.