Science and engineering

At 28, Joseph Henry had at last chosen his career, having rejected the village shop, silversmithing, the stage, medicine and civil engineering in favour of teaching.

After seeing a demonstration of Oersted's discovery of the effect of a current on a magnetic needle, Henry turned to electromagnetism. But as a teacher, rather than a research scientist, Henry had to confine his research work largely to the period of the summer vacation and finances were mostly from his own pocket.

His first significant, indeed major, contribution was the vast improvement he made to electromagnets (which had been invented by William Sturgeon in England). Henry was fascinated. He soon improved the design by adding more turns of wire and insulating the wire rather than the iron core around which it was wrapped, as Sturgeon had done. For a time, before shellac was used, obtaining copper wire and insulating it was one of the repeated, boring problems he had to contend with. At one time his wife's white silk petticoat was shredded to provide the strips of silk he needed.

His first magnet, in 1827, lifted 14 pounds and then 28 pounds compared to Sturgeon's nine pounds. Over the next few years he investigated the best and least expensive ways of making his batteries as well as getting better multicoil magnets. In this work he looked at parallel and series connections and touched on what we would call impedance matching. He came to an empirical understanding of Ohm's Law before he had heard of Ohm and used his own terminology: "intensity" for voltage and "quantity" for current. These terms were adopted by some others and survived for about 30 years.

Soon he was asked to make the first industrial electromagnet, for the Pen-field Iron Works. The site was later renamed Port Henry in his honour. Yale University ordered a magnet in 1831, a monster weighing nearly 60 pounds and which could lift a ton.

The first telegraph?

It was whilst he was teaching at Albany that Henry made what some have called the first electromagnetic telegraph and others the first electric bell.

For a demonstration of the electromagnet he strung nearly a mile of wire around the classroom, more than enough to impress any class of boys. At one end was a small electromagnet which, when energized, repulsed a pivoted permanent magnet so that it struck a small bell. So far as I am aware, there was no suggestion of signalling messages. Henry's greatest scientific discoveries were those of electromagnetic induction and self-induction, both also discovered by Michael Faraday. Faraday is credited with priority for induction and Henry for self-induction. Henry in fact learned of Faraday's work after he had discovered electromagnetic induction himself, but before he had published anything. When the self-effacing Henry did eventually publish in 1832 the final paragraph revealed his discovery of self-induction. Some evidence suggests he found this in 1829; Faraday announced the effect in 1834.