The Poggendorf Mirror | Reviews | world of chemistry

In the second century BCE, the Sicilian-Greek scholar Archimedes provided a theoretical explanation of the action of a lever, the first force amplifier that was one of the key technologies of the ancient world. He showed that two weights would be balanced on a lever arm if their distances from the fulcrum were inversely proportional to their weights. With this, Archimedes introduced a mathematical approach to analyzing the physical world that would transform science across Europe when his books began to be widely read in the 17th century.

The levers could amplify not only the forces, but also their measure. At the end of the 18th century, Charles Augustin Coulomb and Henry Cavendish introduced the torsion method to measure electrical and gravitational interactions. The exquisite sensitivity of the method led to the development of many very precise instruments, especially to measure the earth’s magnetic field which, mysteriously enough, did not align with true north.

In Paris, the French Bureau of Longitude funded a series of scientists and instrument makers to build ever better compasses. This culminated when astronomer Dominique Cassini combined Coulomb’s torsion method with the engraved circles that sea captain and geodesist Jean-Charles de Borda had developed for measuring the meridian of Paris (which defined the metre) . His declination compass, which still exists, is a true marvel of violin making. A silk thread, protected by a glass tube, suspended a magnetic needle inside a box of glass and wood. The frame also contained a small telescope which was used to establish the position of the stars to determine “true” north. But in an ingenious feat, instrument maker Henri-Prudence Gambey added an extra lens to allow the optical system to double as a microscope to read the angle of the Vernier scale on the base.

But its precision made it very fickle to use. In Germany, a young scientist has devised a new approach to making such sensitive measurements.

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Johann Christian Poggendorf, son of a wealthy industrialist, studied for a few years at the famous Johanneum College in Hamburg where Ferdinand Runge was only a few years ahead of him. Poggendorf’s father was ruined when Napoleonic troops besieged the city for five months in 1814, and so Poggendorf was rather unfortunately an apprentice pharmacist for a few years. Runge then advised him to come to Berlin, where he was a student. Poggendorf jumped at the chance and began his studies in 1820. In 1824 he learned that the publisher of Gilbert’s Annals of Physics had died and wrote to the publisher to offer to take over. Amazingly, he got the job and held it for over 50 years, making the journal one of the must-reads of the 19th century.

But he also did science. He improved on one of the first galvanometers, the Schweigger multiplier, in which a coil moved a compass needle enormously within it, inventing the screw-in brass contacts to secure the wires; connectors have become universal. By thinking laterally, he found an ingenious way to amplify the movement of the compass needle: using a mirror as an optical lever, amplifying the tiny twists of the magnet.


He built a simple magnetic declinometer: a brass stirrup held a magnetic rod and a small mirror. The stirrup hung on a silk thread inside a box with a window. With the box in his backyard, he could make observations—accurate to less than 5 arcseconds (one part in 40,000)—from the comfort of his living room, using a small theodolite.

In his article, which he published in his Annalen, Poggendorf apologized for putting something seemingly trivial in the literature. But, he argued, because most of the buildings were iron-riddled, his outdoor device made it very easy to monitor geomagnetic variations in all weathers. Unfortunately, his garden being in the middle of the city, it was impossible for him to make reliable observations due to interference from nearby traffic.

Carl Friedrich Gauss and Wilhelm Weber began using the device in the 1840s. Then William Thomson (later Lord Kelvin) patented a mirror galvanometer in 1858 to measure tiny currents in transatlantic telegraph cables. By shining a light on the mirror and projecting the reflection onto a scale, it became possible to photographically record the movements. Variants of this device would be refined until they reached the limit of Brownian fluctuation in the 1920s. The instruments used in the basement of our department to record spectra in the mid-20th century were so sensitive that measurements could only be made at night when the London Underground stopped.

A few months ago I received an email from an old friend, our long retired Chief Technician, Dave Knapp, a man of wit, skill and almost limitless charm. He had something in his attic that he thought I would like. A few weeks later, a plastic bag filled with bubble wrap was delivered to my office. Inside was the mirror galvanometer he had built as an apprentice in the 1960s, built into a mahogany case. As Archimedes might have said, give me a lever long enough and I will make distant friends cry with joy.


Thanks to Dave Knapp for the gift and Alwyn Davies for the stories of yesteryear

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