The Henry electrical unit – Smithsonian Commemorative Silver Dollar

Today, the Smithsonian Commemorative Silver Dollar shares the tale of Joseph Henry, the first Secretary of the Smithsonian.

On December 19, 1848, Francis Wayland wrote the following to newly appointed Secretary Joseph Henry, “A new impulse will thus be given to investigation in every department of science; and learned men will know that through you they will be able to make their discoveries available to their brethren throughout the civilized world.”

During his lifetime, Joseph Henry made many scientific contributions, perhaps the greatest of which was to electricity.

The Sydney Morning Herald of December 17, 1932 included the following as tribute to Joseph Henry:


Joseph Henry was born in Albany, NY, USA, on December 17, 1797, and died in Washington on May 13, 1878. He made many contributions to science in electricity, electromagnetism, meteorology, capillarity, acoustics, and other branches of physics, and devoted the last 32 years of his life to making the Smithsonian Institution an efficient instrument for “The increase and diffusion of knowledge among men.” Henry was of Scotch descent. His father, of whom little is known, died when Joseph was nine years old. His mother was a woman of refinement, delicate in form and feature, very beautiful, and deeply devotional.

At the age of ten, young Henry was placed in a country store, where he worked for five years; his employer very kindly permitted him to attend school during afternoons. He was fond of reading, especially books of fiction, and, with a highly imaginative mind, loved to amuse the young villagers by relating to them the stories which he had read.

His interest in science was early aroused by finding a small book which a Scotchman had left upon his mother’s table, entitled, “Lectures on Experimental Philosophy, Astronomy and Chemistry.” The influence of this book upon young Henry’s life is indicated by the following entry made in his own handwriting: “This book, although by no means a profound work, has under Providence, exerted a remarkable influence upon my life. It accidentally fell into my hands when I was about 16 years old, and was the first book I ever read with attention. It opened to me a new world of thought and enjoyment, invested things before almost unnoticed with the highest interest, fixed my mind on the study of nature and caused me to resolve at the time of reading it that I would immediately commence to devote my life to the acquisition of knowledge.”

Joseph Henry has been acclaimed the father of the useful electro-magnet, which is undoubtedly the foundation of the electrical industry. Henry’s magnet was used in the discovery of diamagnetism, the effect of magnetism on light, magneto-electricity, and other astounding revelations of the laws of Nature.

Professor J. A. Fleming, of London, writing in 1892, states that Henry’s claim to be an independent discoverer of the fundamental effect of electro-magnetic induction “is not now disputed.” Although Michael Faraday in England made his discovery a few months earlier, Henry had gone further in his discoveries, and the exposition of the laws pertaining to them.

No fundamental advance has since been made over the electro-magnet given us by Henry over 100 years ago. Every electro dynamo or motor uses the magnet in the form in which is was left by Henry in 1829. In fact, every piece of electrical apparatus depends wholly or in part upon Henry’s magnet for its operation, and it is especially interesting, on the 135th anniversary of his birth, to remember that many of his discoveries and inventions, and much of the electrical apparatus which he developed, find their application today in devices used in “wireless telegraphy” and radio apparatus. Our wireless friends will be particularly interested to learn that the “variometer,” so well know in radio equipment, was made possible by Henry’s achievements. He invented the so-called electric relay, universally used in telegraphy and telephony apparatus today.

Practically, all Henry’s remarkable researches and original contributions to science were made while he was a professor at Albany Academy and at Princeton College.

It has become the custom to confer the names of eminent discoverers upon electrical units. For example the name of Volta, who invented the electric pile (the first electric battery) is to be found in the volt—the common unit of electrical pressure. The name of Ampere, who made numerous discoveries in electricity and magnetism, has been given to the unit rate of electric flow, and of Ohm to the unit of resistance after Ohm, who explained the relation between electric pressure, current and resistance. “Farad” has been attached to another unit in honor of the great Faraday, so it is quite fitting to find that the name of “Henry,” given to the unit of electric inductive effect in honor of Joseph Henry, was unanimously adopted by the delegates representing in 1893 not only the scientific societies of America, but of the world then assembled in conference at Chicago.


Joseph Henry continued his scientific study and contributions at the Smithsonian. Unfortunately, the fire at the Smithsonian destroyed many of his scientific papers.

The Smithsonian Commemorative Silver Dollar shows against a statue of Joseph Henry and one of his descriptions of electro-magnetism published in an annual Smithsonian report.

Smithsonian Commemorative Silver Dollar