Marconi Magnetic Detector


This article places Howard Stone among the lucky ones who have found the radio items he wanted "most in the world." Here he shares with us the interesting process of restoring his rare Marconi magnetic detector, as well as information about its history and the theory of its operation. (Editor)

Most seasoned radio collectors spend hours helping antique dealers and others learn about radios. It is in our own interest to have more eyes looking for radio gear. We show them how to determine the age of radios and early wireless apparatus. Is it AC or battery? What type of battery is used? How many prongs do the tubes have? How many tubes? What is the shape of the cabinet? And so forth.

As a teacher at Texas Christian University, every seventh year I am on research leave -- the last two times in Cambridge, England. In England, my wife and I have many friends, among them an antiques dealer who finds items of early technology from time to time. On my last research leave (1992-93), he and I were poring over an early book on electrical and wireless technology while we waited for tea. I would point at a picture and say, "If you ever see this, please buy it for me."

When we came upon a picture of the Marconi magnetic detector and multiple tuner, I remember saying, "I want those two pieces more than any other radio items in the world." I told my friend more about them, and we moved on to other pictures. Tea came, and we put the book aside.

Nine months later, back in Fort Worth, Texas, I received a phone call from England. My friend had found a magnetic detector. Did I want it? My answer, obviously, was yes. When the magnetic detector arrived, it was a little scruffy, but all original. Figures 1 and 2 (see print version) show my prize and its condition. The cover was missing, but that is true of many magnetic detectors. I was pleased to see that it had not been modified. It had a little woodworm damage on the bottom and a sliver of molding had been ripped off, but it was in better shape than I had expected.

The pre-restoration Marconi magnetic detector
Figure 1. The Marconi magnetic detector as it appeared when Howard Stone received it from his antique dealer friend in England.

With a piece of this value, both historical and financial, I was in no hurry to begin restoration. The first few weeks I just examined it, did some research, and slowly began to remove 80 or so years of dirt. After cleaning it, I sent the case to John Kegley for help in its restoration. John also made a cover for it.

The clock mechanism was hardly functioning and very dirty. A magnetic detector is wound up like a clock. I cleaned and oiled it, and it now works very well. The winding key is old, though not original, and came with the detector. It has a first rate mechanism; nothing was spared in quality. The wire loop tension adjustment, shown underneath the right pulley wheel in Figure 2 (see print version), was frozen in position. After two weeks soaking in WD-40, it broke loose.

I reversed the side of the magnets that were facing out, and with some cleaning they looked good. Two wire connections had broken at the lug and had to be soldered. Fortunately, the coils had continuity and did not need rewinding. I replaced the iron wire loop that circles the two spools and goes through the small glass tubes; a short piece of the original wire was still with the detector and helped me in my search for a new piece of iron wire. The restoration was complete and the results can be seen in Figures 3 and 4 (see print version).

The fully restored Marconi magnetic detector
Figure 3. The fully restored Marconi magnetic detector complete with reproduction lid.

Magnetic Detector Background

In 1895, Ernest Rutherford, a scientist in England, was able to detect signals transmittted a mile away, using a magnetometer, a device used for measuring the strength of magnetic fields. Rutherford's research was based on the prior work of Lord Rayleigh and other scientists studying oscillatory currents.

Rutherford attached an antenna to the magnetometer; the received signal deflected the needle of the magnetometer, and thus, an early "magnetic detector" had been discovered. Marconi further developed the work of Rutherford, and in 1902 he developed his magnetic detector. See the accompanying "Magnetic Detector Operation" for more information.

The first "Maggie" (the nickname widely used for the magnetic detector) was installed in the Italian Navy's warship the Carlo Alberto in 1903. Magnetic detectors were very reliable for shipboard communication and were the primary detectors installed on European vessels from 1903 to 1912, replacing coherer type detectors. In fact, because of their reliability, magnetic detectors were used as back-up detectors on many ships into World War I to supplement crystal and later valve detectors. Although wireless technology was developing at a very fast pace at this time in history, because of its dependability, the Maggie had a remarkably long period of usefulness.

Most of the magnetic detectors manufactured were made by Marconi Wireless in England. The ones built in the United States were produced in Aldene, New Jersey. Since the Aldene plant did not begin operations until 1912 when magnetic detectors were on their way out -- being supplanted by valves and crystal detectors -- not many were produced. Today, not more than a half a dozen American versions are known still to exist.

The magnetic detector was very important in the history of radio and most certainly was originally commissioned to a ship. Its reliability, a key to its longevity, and its use on ships at sea no doubt saved many lives.

The magnetic detector described in this article is serial number 31462. It is the pride of my collection. My fondest hope is to acquire a multiple tuner, which was used along with the magnetic detector, in order to complete a typical shipboard receiving set during the very early days of wireless.


The author thanks Mel Bredehoeft, Jonathan Hill, Jim and Felicia Kreuzer, and John Terrey for assistance in the research, and Andy Anderson for the photographs in this article.


Baker, W. J. History of the Marconi Company. New York: St. Martin's Press, 1971.

Blaine, Robert G. Aetheric or Wireless Telegraphy. London: Biggs & Sons, 1903.

Blake, G.G. History of Radio Telegraphy and Telephony. London: Chapman & Hall, Ltd., 1928.

Hammerton, A. Harmsworth Wireless Encyclopedia, Vol. 2. London: Fleetwood House, N. D.

Hill, Jonathan. Radio! Radio! London: Sunrise Press, 1986.

Kreuzer, James and Felicia. "Marconi -- The Man and His Apparatus," The AWA Review, Vol. 9, 1995.

Laughter, Victor H. Operator's Wireless Telegraph and Telephone Hand-Book. Chicago: Frederick J. Drake & Co., 1909.

Rolfe-Martin, A.B. Wireless Telegraphy. London: Adam and Charles Black, 1914.

White, William J., Wireless Telegraphy and Telephony. London: Whittaker & Co., 1912.

(Howard Stone, 2825 6th Ave., Fort Worth, TX 76110)

Howard Stone is a professor of Pastoral Counseling at Texas Christian University, Fort Worth, Texas. A longtime collector, he is especially interested in wireless 1- and 2-tube battery sets of the early 1920s, as well as crystal sets. He has contributed several articles to A.R.C., including a series on British crystal sets and an article on round Ekcos.

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