Amongst the many objects the museum has inherited from the old History of Education Museum collections are over thirty galvanometers, instruments used for detecting and measuring electrical current. Galvanometers work on the principle, first discovered by the Danish physicist Hans Christian Oersted in 1820, that electrical current flowing through a wire will deflect a magnetic needle. Amongst the galvanometers in our collections are several examples of the mirror galvanometer, a much more sensitive version of the original instrument which was first patented by William Thomson, later Lord Kelvin, in 1858.
The mirror galvanometer was developed in response to a pressing practical and commercial need; advances in submarine telegraphy demonstrated to engineers and investors alike that the scientific principles on which the technology was based were not properly understood. This became really important with the attempts to lay a telegraph cable across the bottom of the Atlantic Ocean to connect Great Britain and America in 1857 and 1858, the longest cable ever laid up until that time. The problem was that, over a long underwater cable, signals at the receiving end were very faint and difficult to detect. This was because the discrete electrical impulses became attenuated, stretched out, due to the capacitance, or electrical storage properties, of the long underwater cables. The end was result was that one signal would become stretched out and blurry, and multiple signals sent one after the other would run into one another and all that was detected at the receiving end would be a messy, unintelligible noise.
Two solutions were proposed to this problem. One was, effectively, to push more current through the wire (see here for more). However, the result of this was to burn out the already faulty 1858 cable, only three months and 732 messages after it had been laid (see Charles Bright, The Story of the Atlantic Cable, 1903). Thomson’s solution was instead to design a more sensitive receiving instrument. The mirror galvanometer comprised a small mirror with a magnet fixed to the back, suspended within a coil of wire so that it hung freely in the middle. When the current flowed through the wire, the magnet moved, thus twisting the mirror. A lamp was used to shine a light onto the mirror, and, as it moved, the light was reflected onto a scale set up opposite the galvanometer. This in effect created a weightless pointer. The movement of the light-spot on the scale indicated the presence, and the magnitude, of the current passing through the receiving instrument.
The author Arthur C. Clarke provided an elegant explanation for how this instrument detected such small currents: the initial electrical impulse, he wrote, in Voice Across the Sea, was like water behind the wall of a dam (1974, pg. 46-50). The edge was clearly defined by the vertical line of the wall. However, if the wall broke, the water would immediately begin to flatten out, and would form a wave, the crest of which would form a short distance behind the beginning of the flow of water. Clarke explained that this was similar to the attenuation of the electrical impulse; the first current to reach the receiving instrument from the original electrical impulse would be the equivalent of a trickle before the crest of the wave. The efficacy of Thomson’s mirror galvanometer arose from its ability to detect this initial trickle, without needing to wait for the crest of the electrical wave before registering a signal. Thus, it could rapidly provide separate readings for multiple, consecutive signals, one after another.
The mirror galvanometer gave experimenters a tool for studying and quantifying electrical current which was so accurate that variations on it were used in laboratories for decades afterwards (see Graeme Gooday, The Morals of Measurement, 2004, pg. 137-48). It is thus a good example of an instrument which was devised for a commercial purpose but which then went on to benefit scientific research into electricity. The mirror galvanometer inspired such wonder amongst many of Thomson’s contemporaries that one, the physicist James Clerk Maxwell, was inspired in 1872 to write a short poem about it, parodying some of Tennyson’s verses: “The lamplight falls on blackened walls, and streams through narrow perforations. The long beam trails o’er pasteboard scales, with slow decaying oscillations. Flow, current, flow, set the quick light-spot flying. Flow, current, answer, light-spot, flashing, quivering, dying.” (Clarke pg. 51 or Gooday pg. 148)
Our mirror galvanometers date from the early 1900s, and would have been used in local Yorkshire schools to teach children about physics.