Thursday, October 28, 2010

The Induction Coil (Part 1)

Heinrich Daniel Rühmkorff removed the umlaut from his last name when he moved to England. Despite the timing, it was probably convenience and not politics as neither language uses them. It was a pragmatic decision, a common quality in engineers. He is often credited with the invention of the induction coil. it's not that simple of course, but the writers of history are pragmatic in an entirely different way.We'll get back to Mr. Rühmkorff in Part 2.
The image above is from The Handbook of Electricity in Medicine by W. H. Guilleminot, printed in 1906.  The Ruhmkorff coil is now known as an induction coil, and it was the invention that directly led to the modern electric transformer. Coiled wire behaves very differently under different circumstances. It is different from the Tesla coil (1891), or the Henry coil (1831). The mad scientists of the day liked to generate sparks. They also used the Holtz machine and the Voss machine. These were not props, but all early stages in the development of electronics.None the less these nuts seemed to enjoy shocking each other and themselves.  Let me quote Joseph Henry
"If the extreme poles of this compound arrangement be terminated by the copper handles, and these be brought in contact, holding one in each hand, a deflagration of the metal will be produced, and a thrilling sensation, scarcely supportable, felt in each arm."
The first real induction coil was invented by Charles Grafton Page and patented in 1836. Page had been trying to improve Joseph Henry’s calorimeter [Which was based on Hare's calorimeter.] There are some sources that suggest he was also familiar with Faraday's concept of self-induction but this is apocryphal. Henry developed a number of copper coil experiments. Page was a student at Harvard at the time and was studying the medical application of electro-therapy and took an interest in Henry's idea of fun. 

Page wound a coil of copper ribbon through a series of cups filled with mercury using them as electrical connectors. One terminal of the battery was then connected to the innermost cup of mercury and the other terminal to different cups elsewhere along the length of the coil. I'll quote Wikipedia on how Page liked to shock himself with the resulting product.
"He held a metal wand in each hand, and put these wands into the same two cups as where the battery terminals went — or any other pair of cups. When an assistant removed one of the battery terminals, stopping the current from going in the spiral, Page received a shock. He reported stronger shocks when his hands covered more of the spirals length than where direct battery current went. He even felt shocks from parts of the spiral where no direct battery current passed."
Working concurrently on a similar set of coils was British experimenter William Sturgeon. Sturgeon was familiar with Page and even reprinted Page's article in his journal Annals of Electricity. Sturgeon devised coils that were adaptations of Page's instrument, where battery current flowed through one, inner, segment of a coil, and electrical shock was taken from the entire length of a coil. Page in turn developed a coil  experiment with two wires, a primary and a secondary. The first was a primary coil heavy gauge copper wire wrapped around a bundle of thin iron rods. The second was a coil of fine wire wrapped around the outside of the primary coil with silk insulation in between them. Page  published an account of this which he documented in a paper titled The Compound Electro-Magnet and Electrotome. In the 1840s he worked at the US patent office and he build telegraph equipment for Samuel Morse

Sadly external events drove him out of science.  In 1863 Union soldiers in the trashed his lab and he quit the business. Virginia had joined the Confederacy in 1861 moving the war front to northern Virginia. It's likely his property was trashed in the retreat in the spring of 1863. he never returned to science. He died 5 years later at the age of 56.