You don’t get knighted for nothing. When Scottish-Irish physicist William Thomson was knighted in 1866 (then further honored years later when elevated to the peerage as Lord Kelvin) one of the primary reasons was his groundbreaking work on the mirror galvanometer. His adaptation of this device, which he patented in 1858, allowed telegraph operators to detect the faint signals traveling across the new transatlantic telegraph cables.
Our simple galvanometer tutorial illustrates the electromagnetic principles behind this instrument, which measures current. While other galvanometers use deflecting needles that point to numbers on a scale to measure the current, the mirror galvanometer uses, as its name implies, a mirror.
The tutorial below shows how it works.
The key features of the galvanometer are hidden inside the instrument and not visible in this tutorial. They consist of a wire coil inside of which a tiny mirror is suspended by a silk thread. Attached to this mirror is one or more permanent magnets. When a current runs through the coil, it becomes an electromagnet, the field of which interacts with the permanent magnet’s field in such a way as to cause those magnets to turn – and with them the mirror to which it is attached.
This is where the second part of the device, a calibrated scale, comes in. A lamp is positioned behind the scale; its light passes through an aperture in the scale that is directed at the galvanometer’s mirror. The mirror reflects that light back onto the scale; when the magnet/mirror is deflected by an electric current, the light deflects with it, measured by the scale.
This type of galvanometer is exceptionally sensitive and can detect very weak currents, such as those that made the long journey from Canada to Ireland in the first transatlantic cables. As you can see by manipulating the current slider back and forth, the direction of the current is reflected in the scale. Telegraph clerks of the era knew that readings in one direction signified a dot, while readings in the opposite direction meant a dash.