How the Curie Came to Be

This story appeared in two parts in the October and November 1996 issues of the Health Physics Society's Newsletter.

Part One

Ernest Rutherford could be forgiven for thinking that his main job as Chairman of the Radium Standards Committee was flattering its members and seeming to say one thing while meaning another.

It was a dirty job. But somebody had to do it if the Committee’s mission, the establishment of an international radium standard, was to be accomplished. Rutherford’s problem: Marie Curie would have to be the one to prepare the standard and she would want to retain possession— something most of the Committee’s members would find unacceptable. They would want the standard to become the property of the Committee and be preserved at the Bureau International des Poids et Mesures near Paris. Skids had to be greased.

Fortunately, grease was available. Another of the Committee’s tasks was to straighten out the nomenclature dealing with radioactive substances. Why not create a new unit and name it the curie? Two birds, one stone!

The historical roots of what became the original definition of the curie probably lie in a recommendation made in 1904 by Bertram Boltwood (the U.S. representative on the Committee) that the amount of radon from one gram of uranium in an old mineral be taken as a standard (Badash 1979). However, by 1910 it seemed more logical that the unit be based on the amount of radon in equilibrium with radium rather than uranium. And, if anything, there was even more interest in having a unit that described an equilibrium quantity of radon. As F.A. Paneth (1950) noted, radium already had a unit (weighable quantities were expressed in grams), but radon had none. Paneth argued that a unit was necessary for "radon—a radioelement of at least equal importance [to radium] for medical use—since it was impossible to weigh." However, the gram had limited usefulness as a unit for radium. Only milligram and larger quantities of radium could be weighed accurately. And even when sufficient amounts were available, the radium had to be purified and unencapsulated. Furthermore, in 1910 radon’s main significance lay in its role as a means to quantify radium, not in the medical arena. The small amounts of radium researchers often dealt with were typically quantified by measuring (via electroscope) the radon emanation. In fact, the first working radium standards might have been emanation standards (Badash 1979). In this sense, the curie was as much a unit for radium as it was for radon.

Technically, the Radium Standards Committee was formed and held its first meeting at the International Congress of Radiology and Electricity in Brussels (13-15 September 1910). But, it should come as no surprise that most of the decisions had been worked out in advance by the inner circle: Rutherford and Stefan Meyer (the latter would serve as the Committee Secretary). No fool, Rutherford also discussed the matter with Curie prior to the Congress. But the discussion hadn’t gone exactly as planned and he warned Meyer in a letter (8 September 1910) that Marie wanted the "curie" defined in terms of 1 gram of radium. The committee would not be pleased; a practical unit would be wanted that could be conveniently applied to the quantities normally worked with. In 1910, 1 gram of radium was a "legendary" amount.

On the opening day of the International Congress, Rutherford’s plans were set in motion by a German physicist "unconnected" with the Committee, Eduard Riecke. Translating from the Congress Proceedings: "Professor Riecke (Gottingen) proposed that the name ‘curie’ be given to a radiometric unit" and "Madam Curie (Paris) accepted the proposal of Mr. Riecke, for the honor rendered the memory of Pierre Curie."

The following day, the Radium Standards Committee met for the first time. Despite Marie’s desire that the curie be based on one gram of radium, it was decided that the curie should be the amount of radon in equilibrium with 10-8 grams of radium (Boltwood in a letter to N. Ernest Dorsey, 20 January 1921). But as Boltwood complained later: "Madame Curie was a member of the committee and agreed in this decision [regarding the 10-8 figure] BUT - at an unearthly hour the next morning, she arrived at the hotel where Rutherford and I were stopping and informed us that after thinking the matter over she felt that the use of the name "curie" for so infinitesimally] small [a] quantity of anything was altogether inappropriate."

The Committee’s report had to be presented at the very next session of the Congress. There was no choice. Marie Curie got her way.

Part Two

Image of Marie Curie

At the end of the previous exciting episode, Marie Curie had engineered a last-minute victory in the Radium Standards Committee: the curie would be based on a full gram of radium.

Rutherford reported this decision to the full assembly of the International Congress of Radiology and Electricity on the closing day of the meeting. Unfortunately, all that the Congress proceedings say about the new unit is that the name curie was given to the amount of radon in equilibrium with one gram of radium.

As this decision and those of the other committees were presented, the Congress degenerated into a tower of Babel. Resolutions were thrown forth in German, French, and English. Amendments were added to amendments and the Chairman of the Congress lost any semblance of control. Finally, as recollected by A. S. Eve (the Canadian representative on the Radium Standards Committee), W. Hallachs took over the Chair and in a loud voice "moved the opinion of the Congress, and it was carried!... cosmos out of wild chaos." Thus was born the curie.

The most "official" statement concerning the definition of the curie seems to have been the following comment from Rutherford in an October 1910 issue of Nature (Rutherford 1910): "In the course of the Congress it was suggested that the name Curie, in honor of the late Prof. Curie, should if possible, be employed for a quantity of radium or the emanation. This matter was left for the consideration of the standards committee. The latter suggested that the name Curie be used as a new unit to express the quantity or mass of radium emanation [radon] in equilibrium with one gram of radium (element)."

Does this lay to rest the $64,000 question, "Whom was the curie named after?"—a question that can arouse the passions of health physicists when nothing else will?

Ja! Non! Perhaps!

Almost always, Rutherford restricted his comments to what the facts permitted. His words were reasoned, well thought out. What Rutherford stated in Nature was that a suggestion was made, that this suggestion was to be considered by the Standards Committee, and that the Committee defined the curie as the amount of radon in equilibrium with one gram of radium.

Why would Rutherford make such an ambiguous statement? Perhaps it was because the curie was obviously named after Marie and/or Pierre and because some Committee members would have objected if they thought it was named after Marie. Consider the following comments of Bertram Boltwood (Badash 1969): "Mme. Curie is just what I have always thought she was, a plain darn fool...Meyer is quite right in the position he takes." Rutherford must have created the ambiguity intentionally so that some, like Boltwood, could hold the opinion that it was named after Pierre.

Rutherford’s personal view is found on page 479 of his book Radioactive Substances and Their Radiations (Rutherford 1913): "At the Radiology Congress in Brussels in 1910, it was decided to call this equilibrium quantity a ‘curie’ in honor of M. and Mme. Curie."

With one exception, I couldn’t determine the opinions of the other members of the Standards Committee (M. Curie, F. Soddy, S. Meyer, O. Hahn, H. Geitel, E. Schweidler, A. Debierne, and A.S. Eve). However, I was successful with Arthur Stewart Eve. He states on page 192 of his biography of Rutherford (Eve 1939) that "it was agreed that the amount of radium emanation... should be called one Curie in honor of Madame."

Had Rutherford lived to read Eve’s comment, he probably would have laughed and said something to the effect, "Rutherford my boy, you did your job well!"

It would seem that the curie was named after whomever you choose: Pierre, Marie, or both. Rutherford wanted it that way.

References

  • L. Rutherford and Boltwood - Letters on radioactivity. Yale Univ. Press, New Haven; 1969.
  • Badash, L. Radioactivity in America - Growth and Decay of a Science. Johns Hopkins Univ. Press, Baltimore; 1979.
  • Eve, A.S. Rutherford - Being the life and letters of the Rt Hon. Lord Rutherford. O.M. MacMillan Co., New York; 1939.
  • Rutherford, E. Radium standards and nomenclature. Nature 84 (2136):430-431; 1910.
  • Rutherford, E. Radioactive substances and their radiations. Cambridge Univ. Press, London; 1913.

The kind assistance of Ron Kathren and Lawrence Badash is gratefully acknowledged.