Monday, December 01, 2014

The D'Arsonval Galvanometer

A galvanometer is a specialized type of ammeter. This is a mechanical device for detecting electric current. Despite how mundane that sounds, Jacques Arsene d'Arsonval remains under-rated. He was a French-born physician, physicist and inventor back in an era when you could be all three of those things without being required to earn bogus annual continuing education credits. Somehow without all that professional resume and support he managed to invent a moving-coil galvanometer and the thermocouple ammeter.

He was born in La Porcherie, France in 1851. He was supposed to attend the very prestigious L'Ecole Polytechnique but that was cancelled with the outbreak of war in 1870. He studied medicine at the University of Limoges and then in Paris and earned his degree in 1877. In 1892, he became a lab director in biophysics at the College de France and two years later he was appointed to a professorship.

So about that galvanometer. Basically he took the fishbone galvanometer invented by Marcel Deprez and inverted it so that the deflection was generated by the movement of the coil rather than the iron needles aka "fishbones."  In both units a current passes through the coil but d'Arsonval did away with the iron needles and used an iron tube to shield the unit instead. It was a distinct improvement but the need for local calibration led James Swinburne to criticize it, but Swinburne was a very critical guy.You can read up on his trolling in the book The Morals of Measurement by Graeme Gooday.

D'Arsonval was also a contributor to the emerging field of electrophysiology. That hundred dollar word is refers to the science of zapping animals with electricity. As you may recall that was a big hobby among early electronics enthusiasts. Among them was Italian researcher Luigi Galvani, who discovered in 1791 that when you zap a frog with electric current it's leg jerks. For that moment of comedy genius we name that specific  ammeter a galvanometer.But that does take us back to d'Arsonval. He too enjoyed zapping himself and others with currents of various frequencies. He determined that at the frequency of 10 kHz the current caused the recipient no pain, for that reason in older literature it's called the d'Arsonval frequency.  (It is also called the Tesla Current.) More here.

As limited as the d'Arsonval galvanometer was, Edward Weston was able to improve the design, replacing the wire suspension with a pivot like a hairspring in a clock. By 1888 he patented a commercial model. Modern ammeters strongly resemble this design with the addition of a torsion spring to pull the coil and pointer back to the zero position. At least we got over zapping people for fun.