1. Faraday and Henry Did Similar Induction Experiments on Both Sides of the Ocean
Michael Faraday successfully created an electric current using a device with wire wrapped around an iron ring in late August 1831. His experiment notes recorded that a needle moved and then settled back into its original position. Because historical records vary between August 29 and 31, it is safest to say the success happened at the end of August [2]. At that same time, Joseph Henry was teaching at the Albany Academy in New York State. During his employment from 1826 to 1832, he is said to have created strong electromagnets by tightly winding insulated wire around an iron core and investigated phenomena related to electromagnetic induction [1].
Faraday also mentioned Henry’s electromagnets in his own experimental records, noting that magnets made by Henry and others could lift weights of 2,000 pounds [4]. Although they were far apart, both men used similar tools and asked similar questions. So, what determined which of these two men would be remembered as the discoverer?
Safety Note: Historical dates for Faraday’s success vary. IEEE sources state August 31, while other sources say August 29. It is safer to avoid specifying the exact day and simply refer to late August.
2. Faraday Quickly Announced His Induction Results in Three Places
In science, being the first to publicly announce a discovery is a key clue in determining priority. Faraday was aware of this, writing in his first paper that "the date of scientific papers claiming discovery often becomes a serious matter." This was a general statement, not specifically directed at Henry [4].
Faraday’s paper was read at the Royal Society in London on November 24, 1831, and was printed around April 1832 [3]. Before that, in December of the same year, he sent a letter to the French Academy of Sciences in Paris with his results. On December 26, François Arago introduced this letter to the Academy [2]. Thus, after his successful experiment, Faraday left multiple dated records: an oral presentation, a letter to Paris, and a printed paper. This raises the next question: what was Henry doing during this time?
3. Henry's Induction Experiments Were Paused in 1831 and Resumed in 1832
Joseph Henry began his induction experiments in 1831 but had to pause because the room he wanted to use became unavailable. He resumed the experiments in 1832. He learned of Faraday’s discovery in spring or early summer of 1832, with sources varying between April and June [5].
In July 1832, Henry published a paper titled "On the Production of Currents and Sparks of Electricity from Magnetism" in an American scientific journal. This was about seven months after Faraday’s paper was read at the Royal Society and about three months after it was printed [6]. Near the beginning of this paper, Henry acknowledged that the deep connection between electricity and magnetism had become clear through the prior discoveries of Oersted, Arago, and Faraday [6]. As a school teacher with limited access to a laboratory, Henry may have tried to make up for lost time with the content of his paper. However, this is speculation by the author and not a reason stated in the historical records. So, what did Henry’s paper contain that Faraday’s did not?
4. Henry's Paper Ended with a Fact He Said Was "Not in Any Book"
At the end of his paper, Henry added a note saying, "I state one thing not in any book," and described a specific phenomenon. He explained that if current is passed through a coil of wire 30 to 40 feet (about 9 to 12 meters) long, and the circuit is broken, a strong spark appears. With weaker batteries and shorter wires, the spark was not visible [7].
This is a phenomenon called "self-induction," where a voltage is generated in the coil itself when the current changes. Henry wrote that the spark was clearly visible with long wires but not with short ones. Faraday also independently discovered self-induction and published it in 1834 in the ninth series of his "Experimental Researches in Electricity." Historical sources consider this an independent discovery from Henry’s work [5]. However, Henry’s 1832 paper was the first to describe it. Since Faraday is remembered as the discoverer of induction, where did Henry’s name remain?
5. Why Faraday Is Known for Induction and Henry Is Known for a Unit
In 1893, an International Electrical Congress held in Chicago, USA, decided on names for electrical units. It is reported that the unit for inductance, which describes the property of a coil, was named the "henry (H)." This was 15 years after Henry’s death [9].
One henry represents the property of a coil that generates a voltage of 1 volt when the current changes at a rate of 1 ampere per second. On the other hand, the unit named after Faraday, the "farad (F)," represents the ability to store electricity. The henry and the farad are different units representing different quantities [8]. Additionally, Henry became the first Secretary of the Smithsonian Institution in the United States in 1846 [8]. The rule of assigning the name of a discovery to one person may have made the other person’s contributions less visible in retrospect. However, if a name remains as a unit, people around the world write that name every time they perform electrical calculations.
6. Find the Units Henry (H) and Farad (F) in Science Books
Science textbooks and library books on electricity often include lists of units. Check if the henry (H) and the farad (F) appear in the same table as the volt (V) and the ampere (A). If they are there, read the descriptions to verify what quantity each one represents. Another good method is to borrow two books from the library that cover electromagnetic induction. Comparing how each book describes the discoverer can help you understand the mechanism behind why a discovery’s name is often assigned to just one person.