Tag Archives: physics

Seeing the light – Lord Kelvin’s mirror galvanometer

Mirror galvanometer

One of our mirror galvanometers. In this instrument the mirror apparatus has been removed.

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.

Mirror galvanometer, 1858, Science Museum
Here you can see the lamp and scale required to operate this 1858 mirror galvanometer (Science Museum, London).

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.


Hidden Histories: Edinburgh to Leeds, via South Africa

This attractive piece from the collection of the School of Physics and Astronomy is a differential hygrometer, an instrument used for measuring the humidity of the air.  The dedication plaque on the box announces that it was presented to Charles Piazzi Smyth in 1836.  Piazzi Smyth (1819-1900) would later serve as the Astronomer Royal for Scotland from 1846 until 1888, but at this time he was just 17 years old and working at the Royal Observatory at the Cape of Good Hope as an observatory assistant.

Hygrometers were an important part of an astronomer’s tool kit as they allowed one to calculate the dew point, the temperaturePiazzi Smyth's Differential Hygrometer at which moisture condenses out of the air, leaving it clearer.  This allowed for more accurate and precise observations of stars, the light from which was otherwise distorted by refraction in the atmosphere.  This one probably worked by measuring the difference between the outside temperature, on the standard thermometer, and the temperature recorded on the u-bend thermometer.  This latter reading was attained by keeping the cloth over one of the bulbs wet; the evaporation of the moisture would lower the temperature, and the difference would allow the user to determine the relative humidity of the air by use of a set of tables.

The hygrometer was made by Adie and Sons, Edinburgh, prolific manufacturers of scientific instruments. It is likely that this instrument travelled with Piazzi Smyth to South Africa, and may have been used on his local expeditions around the area.  The two carry handles on the box suggest that it may have been intended to be portable, implying an untold story of a roving instrument used by an astronomer who frequently travelled beyond the confines of the observatory.  How it ended up in Leeds is still uncertain, but, as Piazzi Smyth lived quite close to Leeds during his retirement in Ripon, it is possible that it was either acquired by a member of staff when his possessions were auctioned off after his death, or even presented personally to the department back when it was still part of the federal Victoria University.

UPDATE: You can view the finished video on our YouTube channel here.

Hidden Histories: An ordinary capacitor?

The collection of historical scientific instruments held by the Physics Department at the University of Leeds is eclectic and diverse, ranging from instruments used by William and Lawrence Bragg to a wonderful collection of mechanical calculators and almost everything in between. One of the more obscure objects in the collection is a Siemens Brothers (London) Condenser No. 2 Mark II, now commonly referred to as a capacitor. There is a wide selection of capacitors squirrelled away in the Physics collection and this one would not stand out amongst them but for further information on its provenance.

A large paper label upon the side of this object states this capacitor was tested against standard instruments by the Wireless Testing Department at HMS Vernon, this being somewhat surprisingly not a vessel but a torpedo training school based in Portsmouth. The label is signed and dated although this is very hard to read – the year may be 1905 or 1908 with the latter being more probable. HMS Vernon was also the site of the initial wireless tests done with Marconi wireless apparatus by the Royal Navy in 1899 and hence would play a role in the world’s first commercial wireless contract, between the Marconi Company and the Admiralty.

This surprisingly ordinary object can be used to spin many a tale, of the early development of wireless in Britain or of shared apparatus between cable and wireless telegraphy. But the story I wish to tell with this object is less readily answered – how did this object come to be in the Physics Department at Leeds University? To be sure, condensers were commonly used in Physics teaching and research in the early twentieth century and indeed the Physics Department holds many a condenser. But what I want to talk about when I talk about this object is the transmission of knowledge between the spheres of physics, technology, and commerce. Condensers were an outcome of physical experiments and, through telegraphy and other practical electrical systems, came to be used in a diverse range of technological systems including commercial wireless. And then for some reason, possibly obsolescence, this piece of apparatus is no longer needed and ends up in the Leeds University Physics Department where it is used to teach and possibly form the basic of further experiments used to develop more technologies. And so the cycle of experimentation, innovation, and knowledge transmissions continues.

UPDATED: You can view my short video about the condenser on YouTube here.

Hidden histories update

Thanks everyone who came to the meeting on Wednesday, and I look forward to your posts on your chosen objects.  For those of you not there, this is what we discussed and decided, more or less.

All together we now have at least 9 objects for the case: 2 from physics (and possibly another from maths), 2 from botany/ biology/ herbarium, 1 from history of education, 1 from medicine, 1 from English, pieces from the Newlyn-Philips machine and the Astbury camera.  The plan is now for each person who chose an object to tell us about it in 4 main ways: by talking about it on camera for 1 minute, writing a catalogue entry, writing c.500wds on it on the blog and later putting this on the website linked to the catalogue entry.  And finally to reduce all that to a 30wd label for the case.

Everyone seemed pretty happy with the Hidden Histories theme, each pleasingly interpreting this theme in a different way.  The theme can be taken to mean hidden objects and collections but also hidden stories and the hidden history of the university, which it was generally felt, is not celebrated at Leeds as much as at other universities, but is interesting and a potential selling point of Leeds university.

That’s all, please watch out for (or write and post) information on all the objects so far chosen for the case.