|`WHY DID THEY CALL IT THAT?|
|The Origin of Selected Radiological and Nuclear Terms|
|Paul W. Frame|
|Oak Ridge Associated Universities|
Alpha, Beta, and GammaRadiation
The first reference in the scientific literature to alpha and beta radiation is found in the following statement by Ernest Rutherford (1899): "These experiments show that the uranium radiation is complex and that there are present at least two distinct types of radiation - one that is very readily absorbed, which will be termed for convenience the alpha-radiation, and the other of more penetrative character which will be termed the beta-radiation."
The physicist and historian Alfred Romer (1960) commented that the name "alpha radiation" had been chosen for "no particular reason." While it is true that Rutherford didn’t explain why he chose the first letters of the Greek alphabet, Rontgen had already set a precedent for an alphabetical designation with the name "X-rays," and Rutherford, like many of his generation, had studied the Greek language.
In1902, Rutherford moved on to the third letter of the Greek alphabet when he applied the name gamma rays to the very penetrating radiation described by Villard. Soon thereafter, J.J. Thompson employed the term delta rays, and today we have a veritable alphabet soup of subatomic particles.
E. Rutherford, Uranium Radiation and the Electrical Conduction Produced by it, Philosophical Magazine 47:116, 1899
A. Romer, The Restless Atom Doubleday and Company Inc, 1960.
A unit of cross section equal to 10-24 cm2. More or less equivalent to the physical cross-sectional area of a typical nucleus, but "cross section," as used in the sciences, refers to the probability of a specific interaction, not a physical area.
The following explanation for the origin of the term comes from a report by Holloway and Baker (1944).
"Some time in December of 1942, the authors, being hungry and deprived temporarily of domestic cooking, were eating dinner in the cafeteria of the Union Building of Purdue University. . . In the course of the conversation it was lamented that there was no name for the unit of cross sections of 10-24 cm2 . . . The tradition of naming a unit after some great man closely associated with the field ran into difficulties . . . The "Oppenheimer" was discarded because of its length . . . The "Bethe" was thought to lend itself to confusion because of the widespread use of the Greek letter. Since John Manley was directing the work at Purdue, his name was tried, but Manley was thought to be too long. The "John" was considered, but was discarded because of the use of the term for purposes other than as the name of a person. The rural background of one of the authors then led to the bridging of the gap between the "John" and the "barn." This immediately seemed good, and further it was pointed out that a cross section of 10-24 cm2 for nuclear processes was really as big as a barn. Such was the birth of the barn."
The shed, a less well-known unit of cross section, was used in the late 1940s and 1950s. Much smaller than a barn, the shed had an area of 10-48 cm2
M.G. Holloway and C.P. Baker, Note on the Origin of the Term"Barn" LAMS 523, Sept. 1944.
The SI special name for a unit of activity equal to 1 dps. The becquerel replaced the curie.
In a letter sent to numerous journals in August of 1975, the International Commission on Radiological Units and Measurements (ICRU) stated that the General Conference of Weights and Measures had adopted the ICRU's recommendation that the SI unit of activity take the name the becquerel. The ICRU explained: "Antoine Henri Becquerel (1852-1908) discovered radioactivity in 1896 ("rayons de Becquerel") and was given the Nobel Prize in Physics in 1903 together with Marie and Pierre Curie." Since the Curies and Becquerel shared the first Noble Prize awarded for work with radioactive substances, it might be considered appropriate that it was the becquerel that superseded the curie.
In the May-June issue of the Health Physics Society Newsletter (1974), one year prior to the ICRU announcement, Keith Schiager proposed that the name "becquerel," abbreviated Bq, be adopted as the SI unit equivalent to one transformation per second. Schiager's intent was to honor the discoverer of radioactivity, Henri Becquerel. If the ICRU was aware of Schiager's recommendation, which seems likely, they don't appear to have acknowledged it.
L. Taylor Organization for Radiation Protection, The Operations of the ICRP and NCRP DOE/TIC 10124, 1979.
A unit of activity equal to 3.7 x 1010 disintegrations per second . The curie has been replaced by the becquerel.
The original intent of the Standards Committee that defined the curie was for it to be based on a smaller activity, similar to those routinely employed in the laboratory. But Marie Curie had other ideas. If it was to bear the name Curie, it had to be large (Badash 1969)!
In the October 1910 issue of Nature, Ernest Rutherford, who chaired the Standards Committee, said "In the course of the Congress it was suggested that the name Curie, in honor of the late Prof. [Pierre] Curie, should if be possible, be employed for a quantity of radium or the emanation [radon]. 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 in equilibrium with one gram of radium (element)."
Note that Rutherford did not say that the committee agreed with, or adopted, the suggestion that the unit be named after Pierre Curie. Three years latter, Rutherford (1913) wrote: "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.
For a more complete and detailed explanation, refer to the
story "How the Curie Came
References L. Badash Rutherford and Boltwood – Letters on Radioactivity, Yale Univ.
Press, New Haven, 1969. E. Rutherford, Radium Standards and Nomenclature, Nature 84 (2136)
430-431, 1910. E. Rutherford, (Radioactive Substances and Their Radiations, Cambridge
University Press, 1913.
L. Badash Rutherford and Boltwood – Letters on Radioactivity, Yale Univ. Press, New Haven, 1969.
E. Rutherford, Radium Standards and Nomenclature, Nature 84 (2136) 430-431, 1910.
E. Rutherford, (Radioactive Substances and Their Radiations, Cambridge University Press, 1913.
Versions of the Cutie Pie, the first survey meter to employ a pistol grip, appear to have been in use in 1944 at both Oak Ridge National Laboratory (then known as the Clinton Laboratory) and Hanford. According to K. Z. Morgan, he first saw a Cutie Pie two months or so after his arrival at Oak Ridge National Laboratory - this would place its use there in 1944 (Stabin 1998). According to Howell et al (1988), the first Hanford Cutie Pie instruments were designed and constructed in early 1944. The basis for this claim is not known. In both places, its primary use would have been to measure the intense radiation fields associated with reactors.
A widely circulated story claims that the name Cutie Pie derives from the formula, Q t p where Q refers to the collected charge within the chamber, and t p refers to "two pi" - the solid angle viewed by the detector. Alternatively, the t is sometimes said to refer to time. In my opinion, this explanation is a bit fanciful and overly convoluted.
Another story is that the name was derived from the alpha-numeric designation given to many Manhattan District documents: CP-XXX. The CP, which referred to "Chicago Pile," presumably became "Cutie Pie."
Nevertheless, the best evidence regarding anything is usually the contemporary literature. In this case, it is the declassified Manhattan District report MDDC-997 "Cutie Pie, A Portable Radiation Instrument", written in September of 1945, by C.O. Ballou. Ballou, an employee at the Clinton Laboratories, describes in detail the reasons behind the development of the Cutie Pie. Among other things, an instrument was needed that was capable of holding its calibration and of being zeroed regardless of humidity conditions. As Ballou noted, "humidity conditions at Clinton run at saturation during the night."
Amazingly, and this is more than one could hope for, Ballou also gives the explanation for the name: "The instrument has been named "Cutie Pie" due to its diminutive size." Most of the other survey instruments that were available during the war were big and heavy, primarily due to the batteries. Since the Cutie Pie only weighed four pounds, picking it up for the first time might well have drawn the reaction: "Hey, that's a real cutie pie!" - a common phrase at the time to describe something really neat.
The Manhattan District Report MDDC-1059 (1947) includes a photograph of a Cutie Pie and states "this instrument has just been declassified and was developed at Clinton Laboratories."
C.O. Ballou. "Cutie Pie," A Portable Radiation Instrument. MDDC-997. Sept. 22, 1945
H.U. Fisher. Instrument Work in an Atomic Energy Laboratory. MDDC-1059. June 17, 1947.
W. Howell et al, Historical Review of Portable Health Physics Instruments and Their Use in Radiation Protection Programs at Hanford 1944 through 1988.
M. G. Stabin, A Window into Health Physics History, Health Physics Society’s Newsletter, July 1998.
In late 1938, Lise Meitner and Otto Frisch determined that it was energetically possible for uranium atoms to split in two when struck by neutrons. To observe this phenomenon, Frisch, working in the basement of Niels Bohr's Institute for Theoretical Physics in Copenhagen, placed a piece of uranium next to the chamber of a proportional counter and exposed the uranium to neutrons. Using an oscilloscope, Frisch looked for the extremely large pulses coming from the detector chamber that would indicate the energetic fragments of the splitting atoms. Among the inhabitants of the Institute who went down to the basement to see what Frisch was up to was the biologist William Arnold (Arnold 1996; Ermanc 1989. At first, all Arnold saw on the oscilloscope screen was a series of small pulses produced by uranium's alpha particles. But then, at Frisch's suggestion, he picked up a neutron source by the handle and put it next to the uranium. The world changed! Huge pulses began appearing - pulses far larger than anything produced by the alpha particles - pulses produced by the fragments of the splitting uranium nuclei! Later that day, Frisch tracked Arnold down and said something to the effect "You're supposed to be some kind of biologist. What is the term you use to describe dividing bacteria?" Arnold replied, "Binary fission." Frisch then asked if the word "fission" would suffice and Arnold agreed that it would.
Arnold, W. Personal communication; 1996.
Ermenc, J. Atomic Bomb Scientists. Meckler; Westport; 1989.
Frisch, O. What Little I Remember. University Press; Cambridge; 1979.
The SI special name for the unit of absorbed dose, equal to 1 J/kg. The gray replaced the rad
In a letter sent to numerous journals in August of 1975, the ICRU stated that the General Conference of Weights and Measures had adopted the ICRU's recommendation that the SI unit of absorbed dose take the name the gray. In an explanatory note, the ICRU commented: "Louis Harold Gray (1905-1965) made one of the most fundamental contributions to radiation dosimetry, the principle now known as the Bragg-Gray Principle." It probably didn’t hurt that Gray had once served as Vice-Chairman of the ICRU.
L. Taylor Organization for Radiation Protection, The Operations of the ICRP and NCRP DOE/TIC 10124, 1979
Health Physics refers to the field of radiation protection. How appropriate the name is has been a matter of some debate (Taylor 1982).
The term Health Physics originated in the Metallurgical Laboratory at the University of Chicago in 1942, but it is not known exactly why, or by whom, the term was chosen. Most likely, the term was coined by Robert Stone or Arthur Compton. Stone was the head of the Health Division, of which Health Physics was one of four sections. Arthur Compton was the head of the Metallurgical Laboratory.
Because the first task of the Health Physics Section was to design shielding for the reactor (CP-1) Fermi was constructing, the original HPs were mostly physicists who were trying to solve health-related problems. This is the crux of the following explanation given by Robert Stone (1946): "The term Health Physics has been used on the Plutonium Project to define that field in which physical methods are used to determine the existence of hazards to the health of personnel."
A slight variation on this explanation was given by Raymond Finkle, an early Health Division employee (Hacker 1987): "the coinage at first merely denoted the physics section of the Health Division . . . The name also served security: "radiation protection" might arouse unwelcome interest; "health physics" conveyed nothing."
The first group of health physicists at the Metallurgical Laboratory consisted of Ernest Wollan, Carl Gamertsfelder, Herb Parker, Karl Morgan, James Hart, Bob Coveyou, and Louis Pardue. John Rose is sometimes added to the mix.
The above photo, taken by Jim Berger, includes most of the original group who used the title "Health Physicist." From left to right: Jim Hart, Ernest Wollan, K.Z. Morgan, Herb Parker, Carl Gamertsfelder and Bob Coveyou.
B. Hacker, The Dragon=s Tail, Univ. Of California Press, 1987.
R. Stone, Health Protection Activities of the Plutonium Project, Proc. Am. Phil. Soc. 90(1); 1946.
L. Taylor, Who is the Father of Health Physics? Health Physics 42: 91-92; 1982.
K-25 was the name given to the gaseous diffusion facility that was built in Oak Ridge during WWII to enrich uranium. At the time, it was the largest building under a single roof - some 47 acres. The K in the name probably derives from the fact that the facility was constructed by the Kellex Corporation, a subsidiary of the Kellogs construction Company. The 25 was a WW II code name for U-235, the desired product of the enrichment process. The code was the combination of the last digit of the nuclide's atomic number (92) and the last digit of its atomic mass number (235). Similarly, U-238 was sometimes referred to as 28, and Pu-239 (atomic number 94) was referred to as 49 (Serber 1992). Of course, the name K-25 might have signified absolutely nothing and simply served as a code name for security purposes. The latter is the opinion of the authors of City Behind a Fence (Johnson and Jackson 1981).
A very common explanation is that K-25 was the facility's location on a map. The same explanation is given for the names of the other two major WW II Oak Ridge facilities: X-10 and Y-12. A problem with this explanation is that X-10 is physically closer to K-25 than Y-12. Furthermore, given the security requirements during WW II, it is unlikely that a potential target for enemy attack would be identified by map coordinates.
Johnson, C.W., and Jackson, C.O., City Behind a Fence, Univ. of Tennessee Press, 1981.
Serber, R. The Los Alamos Primer, The First lectures on How to Build an Atomic Bomb, University of California Press, 1992.
Although the official and legal term for the atomic bomb project was the "Manhattan District," more often than not it was referred to as the Manhattan Engineer District (MED). The Manhattan Project was a popular variation on the official terminology.
Since the Army Corps of Engineers had the overall responsibility for the atomic bomb project, and since the Corps was divided into Districts, it was natural that the organization being established within the Corps to manage the project would be designated as a new District. These districts were normally named after the city in which they were located. The man initially responsible for establishing the organizational structure of the project, Colonel James Marshall, was head of the Syracuse District.
The term Manhattan District was chosen during a meeting between Marshall and Colonel Leslie Groves, who would later take over administrative control of the project. As Groves described it (1962): "After some discussion, during which we considered the possibility of using "Knoxville," we decided upon "Manhattan," since Marshall's main office would at first be in New York City."
The office Groves referred to was at 270 Broadway, very close to New York’s City Hall, and across the street from a park in which there is a memorial to Marie Curie. Within months, the Manhattan District headquarters moved to a building (in Oak Ridge, Tennessee) known as the Castle - the latter name derived from the fact that the Corps of Engineers uses a Castle as its symbol.
At first, Marshall argued that the project should be known as Development of Substitute Materials (DSM), but Groves vetoed this name because he felt that it would draw too much suspicion. Despite Groves' objections, the atomic bomb project was known, at least for a period of time, as both DSM and the Manhattan District.
L. Groves, Now it Can be Told, Harper and Row, 1962.
A unit of absorbed dose. The rad replaced the rep but it has now been replaced by the gray
The name rad was adopted by the ICRU in 1953 at the Seventh International Congress of Radiology. There appears to have been no documented discussion regarding the use of the name prior to this meeting, even though it was determined at the previous ICRU meeting in 1951 that there was a need for such a unit. Aside from the use of rad some three decades earlier as a unit relating to mouse tumors, the rad seems to have made its first appearance in the ICRU report of the 1953 meeting. The reason for selecting the name was not given, nor is it explained in subsequent ICRU, ICRP, or for that matter, NCRP reports.
There is a widespread belief that rad is an acronym for "radiation absorbed dose." This seems reasonable since many other contemporary units (the reb, rep and rem) were acronyms. However, if rad were an acronym, one would expect the ICRU to have identified it as such - something they did not do. If, on the other hand, the rad was just a convenient and concise name, there would be no reason for the ICRU to have explained it. The lack of any explanation in the official literature for the name is totally inconsistent with the idea that rad is an acronym.
This issue has been addressed explicitly by Dr. Lauriston Taylor, Chairman Emeritus of the ICRU (1990). In a 1982 article in the ICRU News Dr.Taylor states: "The term rad was simply suggested as a word by itself. Since then it has frequently been improperly referred to as an abbreviation for "radiation absorbed dose." This is simply incorrect."
L. Taylor, 80 Years of Quantities and Units - Personal Reminiscences. ICRU News June 1990
A military term from the late 1940s. An "official" explanation (Campbell, 1950) is as follows: "The field of radiological instrumentation, as it applies to the defense establishment, has been assigned the name "RADIAC". This new word for the military vocabulary has been taken from the initial letters of - RAdiation Detection, Indication, And Computation."
Like many so-called acronyms, this one is a bit too convoluted to be believable, especially since the first five letters of RADIAC are the same as those in the word radiation. Undoubtedly, the word came first and the explanation followed.
D.C. Campbell, Radiological Defense, Volume IV, Armed Forces Special Weapons Project, 1950.
Radiation (or Radiological) Safety Officer (RSO)
"Radiological Safety Officer" was the title given to the military officer who was responsible for radiological safety during the U.S. atomic weapons tests in the Pacific during the late 1940s.
The earliest use of this term that I know of was in Joint Task Force 7, the group established in 1947 to oversee Operation Sandstone at Enewetak. The regulations for Operation Sandstone read in part:
"Permissible radiological exposure is established at 0.1 roentgens per twenty-four (24) hours. Under unusual circumstances, the Scientific Director and the Radiological Safety Officer may authorize a total exposure up to three (3) roentgens."
As Barton Hacker observed (1994), the regulation’s careful wording was chosen to accommodate the sometimes conflicting needs of the military and scientific participants in the atomic tests. The military, represented by the Radiological Safety Officer, insisted on the ultimate authority for radiological safety, but the civilian scientists, represented by the Scientific Director, frequently needed to enter contaminated areas to perform their experiments and make radiation measurements.
The physicist Karol Froman served as Joint Task Force 7’s Scientific Director, and Col. James Cooney of the Army Medical Corps served as the Radiological Safety Officer. Cooney’s position was essentially the same as that of Col. Stafford Warren in Joint Task Force 1 during Operation Crossroads at Bikini in1946. Warren had been known as the Radiological Safety Advisor.
A distinction was sometimes attempted between the military’s radiological measurements performed for the purpose of safety, and the civilian radiation measurements performed for scientific purposes. Long after the atomic tests were completed in the Pacific, the military tended to favor the word "radiological" over "radiation."
B. Hacker, Elements of Controversy Univ. of California Press, 1994
A unit of the quantity dose equivalent . The rem replaced the reb but the rem has now been superseded by the sievert.
Although the term rem was used as early as 1945, it first appeared in the literature in 1950 (Parker). In this paper, Herbert Parker explained what the rem and another unit of his, the rep (predecessor of the rad), stood for: "The rep is an abbreviation of roentgen equivalent physical. The rem is an abbreviation of roentgen equivalent man or mammal. The more obvious choice of reb (roentgen equivalent biological) is avoided because of the confusion in speech between rep and reb."
Ron Kathren (1986) explains Parker's comment regarding a "confusion in speech" as follows "The unit [the rem] was originally called the reb (roentgen equivalent biological), but during one of his early presentations of the new unit, Parker was suffering from a cold, which led to difficulty in differentiating it from the rep. Accordingly, the name of the unit was changed to rem."
R. Kathren, Herbert M. Parker, Publications and Other Contributions to Radiological and Health Physics Battelle Press, 1986
H. Parker Tentative Dose Units for Mixed Radiations, Radiology 54, 1950
"Scram" refers to the sudden shutdown of a reactor, usually by the insertion of control rods into the core. Also referred to as a "trip."
The term appears to have been coined by Volney Wilson at the University of Chicago during World War two. Wilson was in charge of the instrumentation at Chicago Pile one (CP-1) when Enrico Fermi and his coworkers achieved the first controlled self-sustaining nuclear chain reaction. In particular, Wilson oversaw the construction of the pile’s control rods.
Leona Marshall Libby, the only woman present at CP-1’s initial criticality, had the following to say about the initial use of the term scram: "the safety rods were coated with cadmium foil, and this metal absorbed so many neutrons that the chain reaction was stopped. Volney Wilson called these "scram" rods. He said that the pile had "scrammed," the rods had "scrammed" into the pile." (1979)
Scram is often said to be an acronym for "safety control rod axe man." A common variant, "safety control reactor axe man," is far less plausible because the word "reactor" was not in use at the time (i.e., the mid 1940s). The "axe man" being referred to is Norman Hilberry who stood by with an axe ready to cut a rope tied to the railing of the balcony overlooking the pile. At the other end of the rope was an emergency control rod. If the chain reaction got out of control, Hilberry was supposed to shut it down by cutting the rope and allowing the control rod to fall by gravity into the pile.
A problem with this explanation is that we have Libby's account that the word scram was applied to the process of shutting down the reactor or to the control rods. But there is no record that the term scram was used, at the time the pile was being constructed or at the time of the first criticality, to refer to an individual. Almost certainly, the "safety control rod axe man" story was developed after the fact as a humorous way to explain the origin of a newly invented word that lacked any other convenient explanation.
In a letter written to Dr. Raymond Murray (January 21, 1981), Norman Hilberry wrote: "When I showed up on the balcony on that December 2, 1942 afternoon, I was ushered to the balcony rail, handed a well sharpened fireman's ax and told that was it, "if the safety rods fail to operate, cut that rope." The safety rods, needless to say, worked, the rope was not cut . . . I don't believe I have ever felt quite as foolish as I did then." More to the point, he also stated "I did not get the SCRAM [Safety Control Rod Ax Man] story until many years after the fact. Then one day one of my fellows who had been on Zinn's construction crew called me Mr. Scram. I asked him, "How come?" And then the story." Regarding Leona Marshall Libby's account of the first criticality, from which her previous quotes were taken, Hilberry states " I have not yet had a chance to read Leona Wood Marshal Libby's book, but if she describes that day as I'm sure she must do, her's should be the most authentic recounting on record."
Carl Gamertsfelder, a health physicist who was present at the initial criticality, said that they joked that scram was what you did if there was a problem with the pile (personal communication with Ron Kathren).
L. Marshall Libby The Uranium People, Crane, Russak & Co., 1979.
The SI special name for the unit of dose equivalent equal to 1 J/kg . The sievert replaced the rem.
In the same letter in which the ICRU described its adoption of the name gray (after former ICRU Vice-chairman Harold Gray) as a unit of absorbed dose, they also stated : "The formulation (for selection) of a suitable unit for dose equivalent is therefore a special problem which will be thoroughly discussed by ICRU."
Before the ICRU completed such a discussion, the ICRP (ICRP 1977) selected the name sievert for the unit of dose equivalent. They did so in honor of former ICRP Chairman, Rolf Sievert, whose research focused on the biological effects of the low doses of radiation received by workers and the public. During the latter part of his life, Sievert devoted most of his attention to radiation protection issues.
International Commission on Radiological Protection, ICRP Publication 26 Recommendations of the International Commission on Radiological Protection, Pergamon Press 1977.
L. Taylor, Organization for Radiation Protection, The Operations of the ICRP and NCRP DOE/TIC 10124, 1979
X-10 was the name given to what is now Oak Ridge National Laboratory. The X probably derives from the fact that Oak Ridge was site X during WWII. Los Alamos was site Y and Hanford was site W. The 10 in X-10 might simply refer to the meaning of the Roman numeral X.
Of course, the name might have signified absolutely nothing, and simply served as a code name for security purposes. The latter is the opinion of the authors of City Behind a Fence (Johnson and Jackson 1981).
A very common explanation for the name is that X-10 was the facility's location on a map. The same explanation is given for the names of the other two major WW II Oak Ridge facilities: K-25 and Y-12. A problem with this explanation is that X-10 is physically closer to K-25 than Y-12. Furthermore, given the security requirements during WW II, it is unlikely that a potential target for enemy attack would be identified by map coordinates. Finally, is it simply coincidence that the first facility built at "Site X," a code name for Oak Ridge, was X-10?
Johnson, C.W., and Jackson, C.O., City Behind a Fence, Univ. of Tennessee Press, 1981.
In the following sentence from his paper Ueber eine neue Art von Strahlung [On a New Kind of Ray] (December 28, 1895), Rontgen used the term "x-rays" for the first time in print:
"A piece of sheet aluminum, 15 mm thick, still allowed the X-rays (as I will now call the rays, for the sake of brevity) to pass . . . I find the justification for using the name "rays" for the agent emanating from the wall of the discharge apparatus in the very regular formation of shadows that are produced if one brings more or less transparent materials between the apparatus and the fluorescent screen (or the photographic plate)."
Brevity is the operative word in Rontgen's explanation for his choice of the term. Unlike some of his contemporaries (e.g., William Crookes), Rontgen was not one for flowery language. Designating the rays with a single letter was more his style.
Certainly Rontgen needed to select a name to distinguish these new rays from the other rays associated with gas discharge tubes (e.g., cathode rays and Lenard rays), but why did he chose the letter X? Why not call them Y-rays or Z-rays? The most common explanation is that Rontgen chose the term X-rays to indicate that these rays were of an unknown nature. Although I know of nothing he wrote to suggest this, it makes sense - the letter x is used in mathematics to identify the principle unknown quantity.