The Mirror Galvanometer

A highly sensitive device for detecting current The Mirror Galvanometer was invented by William Thompson (later Lord Kelvin) as a more sensitive way of reading the very small currents on long submarine cables.

A mirror galvanometer consisted of two main units - the galvanometer in a protective housing, and a combined lamp and calibrated scale. These were arranged facing one another.

How it worked

From a lamp fixed below a calibrated scale a beam of light is focused onto a tiny mirror inside the galvanometer. When a current (the incoming signal) passes through the galvanometer coil it temporarily becomes an electromagnet. The magnetic field makes the iron frame, to which the tiny mirror is fixed, move by a tiny amount.

The beam of light reflected off the mirror is therefore deflected to the left or the right, depending on the polarity of the signal. This can be read off against the scale.

The galvanometer with the cover removed.

What the telegraphers would have seen at the other end of the line.

This Galvanometer was used as a receiver by Lord Kelvin at Brest Harbour, France in 1879.

Without the sensitivity of the Mirror Galvanometer, submarine telegraphy would not have been possible, since no other instruments at the time could have detected the current.

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