home | index | units | counting | geometry | algebra | trigonometry & functions | calculus analysis | sets & logic | number theory | recreational | misc | nomenclature & history | physics

# Trivia

Homo sum, humani nihil a me alienum puto.
I am a human being,  I think nothing human is alien to me.
Terence [Publius Terentius Afer (c.190-159 BC) former Phoenician slave]

### Related Links (Outside this Site)

FunTrivia, the Trivia Portal (nonmathematical)   |   The House of Toucans

### Miscellaneous Fun Facts

"Puzzles" [trivia, really] by John Baez.
Notable Properties of Specific Numbers by Robert Munafo.
US Mint   |   US Bureau of Engraving and Printing
What's All This Measurement Stuff, Anyhow?   by  Robert A. Pease.

## Trivia

(2003-12-03)   What is the oldest open mathematical problem?

There are  two  unsolved mathematical problems which date back at least to the times of Euclid  (c.325-275 BC).  Namely:

• Find an  odd  perfect number, or prove that none exist.
• Are there infinitely many twin primes?
(Twin primes are pairs of prime numbers whose difference is 2.)

A positive answer to the latter question is the  Twin Prime Conjecture.

(2003-06-13)   The "North Magnetic Pole" of the Earth...
How is the north-south polarity of magnets defined?  What about Earth?

People have always called  north  the part of a small magnet (or compass needle) that  points northward  (we still do).

The north pole of a magnet seeks the south pole of another...  The idea was first proposed by Sir William Gilbert (1544-1603) [William Gylberde of Colchester] in De Magnete (1600) that the Earth is like a giant magnet whose magnetic south is somewhere up north...  Nevertheless, the common (misleading) usage is to keep calling "Magnetic North Pole" or, better, "North Magnetic Pole" (best capitalized) whichever magnetic pole of the Earth is near its geographic north pole, although this is, technically, the south side of the Earth's magnetic field.

 In modern terms, magnetic induction  (B)  is defined so that  (loosely speaking)  the pseudovector B points away from the north pole and toward the south pole outside of a magnet.  Inside the magnet, it goes from south to north.

The magnetic poles of the Earth are notorious wanderers.  In May 2001, the North Magnetic Pole was located at  81.3°N  110.8°W, moving northwest in the Canadian Arctic at about 40 km per year (more than 1 mm/s).  If the Ross coat of arms is to be believed, the Magnetic Pole was first found on June 1, 1831, at  70.09°N  96.78°W.  Thus, it moved a distance of about 11.65° of a great arc in 170 years  (1295 km).  For a straight motion, that would be an average of  7.6 km per year  (21 meters a day,  87 cm per hour,  0.24 mm per second).

Over geological periods of time, the magnetic field of the Earth has reversed its north-south polarity many times with respect to the Earth's axis of rotation:  It flips once every 400 000 years or so...  The same thing happens with the magnetic field of the Sun, only much more frequently.

Polar Shift on NBC News (January 2011)

(2003-06-13)   From the north side, positive means counterclockwise:
How the right-hand rule determines the sign of axial quantities.

The mnemonic device pictured at left is useful to identify the polarity of the magnetic field created by a simple electrical circuit:  Arrows drawn at the extremities of the capital letters "N" or "S" indicate the direction of the current, when you're watching either the North or South side of the circuit.  This engineering convention is (fortunately) compatible with the traditional definition of the north pole of a magnet, as described above.

 N (+)

Happily, the above mnemonic trick also describes the way the Earth rotates (counterclockwise, if you are above the geographic north pole).  By analogy, astronomers and others call north the side of a rotating celestial body from which it is seen to rotate counterclockwise (regardless of whatever magnetic field may prevail around such a body).

Mathematically, a rotation is a pseudovector  (whose magnitude is the angular speed  w )  directed northward along the axis of rotation.  This states the same convention as the so-called "right hand rule":  Fingers of a right hand show the direction of motion in a rotation whose vector is in the direction of the thumb.

The prefix pseudo- is often used to denote a quantity which is so defined that its sign would change if our notions of "left" and "right" where reversed (the rotation pseudovector is one example).  Such quantities may also be called axial, in contradistinction to the more familiar polar quantities, whose definitions do not depend on any particular convention concerning "space orientation".

The cross-product of two [polar] vectors is axial, so is the cross-product of two axial vectors.  However, the cross-product of a polar vector and a pseudovector is polar.  For example, the relation  V = W´R  holds, which gives the velocity V [a polar vector] for a point of a rotating solid, if W is the [axial] vector of rotation, and R is that point's position [another polar vector] with respect to some motionless origin located anywhere on the axis of rotation...

(2003-12-02; anonymous query)   What initiates the wind?

It's the "tail" of the Sun.

Well, they were right...  Meterological details may be extremely complex, but the wind is caused by differences in pressure which ultimately come from heat produced by the Sun's rays.

On coastlines for example, breezes are felt up to 50 km inland  (landward during the daytime and seaward at night)  which are due to the fact that the Sun's radiation changes the temperature of land masses faster than it changes the temperature of the Ocean.  As a result, the land is warmer than the sea by day, and colder by night.

Global and large scale patterns are also due to the Sun but are less obvious:

Pattern of Prevailing Winds at the Surface of the Globe
NameLatitudePrevailing Direction
North PoleVariable (high pressure)
Polar EasterliesNorthern CircumpolarFrom NE to SW
NorthernVariable (low pressure)
WesterliesMidnorthernFrom SW to NE
Horse LatitudesAround 30°NVariable or calm (high pressure)
Northeast Trade WindNorthern TropicalFrom NE to SW
DoldrumsEquatorialVariable or calm (low pressure)
Southeast Trade WindSouthern TropicalFrom SE to NW
Horse LatitudesAround 30°SVariable or calm (high pressure)
Roaring FortiesMidsouthernFrom NW to SE
SouthernVariable (low pressure)
Polar EasterliesSouthern CircumpolarFrom SE to NW
South PoleVariable (high pressure)

A dubious legend surrounds the naming of "horse latitudes":  A sailboat could be becalmed there for so long that the horses it carried would have to be thrown overboard, in order to save fresh water for people...

The term "wind" is best reserved for the flow of air near the surface of the Earth.  At higher altitudes, the term "current" is preferred.  The prevailing high-altitude currents always blow  from the west  (the strongest such steady current is known as the  Jet Stream).  The prevailing easterly winds  (the tropical trade winds and the polar easterlies)  are thus relatively shallow...

The direction and intensity of the wind is often dominated by travelling cyclonic or anticyclonic meteorological disturbances  (especially in the zones indicated as "variable" in the above table).

cyclone  is a low pressure zone, around which winds rotate counterclockwise in the northern hemisphere (clockwise in the southern hemisphere).

An  anticyclone  is a high pressure zone, around which winds rotate clockwise in the northern hemisphere (counterclockwise in the southern hemisphere).

kokapelli (Indianapolis, IN. 2000-09-07)
In algebra, why is the letter "m" a symbol for slopes in linear equations?
prisonin (2000-11-10)
Why is slope "m", in the "slope-intercept" form of a linear function?
1621 (2002-04-29)
Why is "m" used for slopes?     [As in:   y = m x  + b ]
BooBooLuvsU (2002-04-02)
Why is "m" used to represent the slope in a linear equation?

Well, the explanation is certainly not the one most often given, namely that "m" is the first letter of the French verb "monter", meaning "to climb"; I happen to know first-hand that virtually all French textbooks quote the generic linear function as  y = ax+b.  If the tradition was of French origin, wouldn't the French use it?

In an earlier forum on this [apparently popular] subject, John H. Conway rightly called the above explanation an "urban legend".  He half-heartedly put forward [and later half-heartedly recanted] the theory that what we now call "slope" was once better known as "modulus of slope" ("modulus of..." has often been used to mean "the parameter which determines...").  In 1990, Fred Rickey (of Bowling Green University, OH) could not even find any use before 1850 of the word "slope" itself to denote the tangent of a line's inclination...

Conway "seemed to recall" that Euler (1707-1783) did use m for slope, which remains unconfirmed.  However, Dr. Sandro Caparrini (University of Torino) found out that at least one contemporary of Euler did so, since Vincenzo Riccati (1707-1775) used the notation y = mx+n as early as 1757, in a reference to Jakob Hermann (1678-1733).  (This and other related facts have been reported online in the excellent historical glossary of Jeff Miller; look under Slope.)

Eric Weisstein reports that the use of the symbol m for a slope was popularized around 1844 [A Treatise on Plane Co-Ordinate Geometry, by M. O'Brien.  Deightons (Cambridge, UK) 1844] and subsequently through several editions of a popular treatise by Todhunter, whose notation was y = mx+c. [Treatise on Plane Co-Ordinate Geometry as Applied to the Straight Line and the Conic Sections by I. Todhunter, Macmillan (London, UK) 1888].

The preferred notation for the slope-intercept cartesian equation of a straight line in the plane is not at all universal.  Here's what I have gleaned so far:

 y = m x  + n Vincenzo Riccati (1757).  Netherlands, Uruguay. y = m x  + c UK. y = m x  + b US, Canada. y = a x  + b France, Netherlands, Uruguay. y = k x  + b Russia. y = k x  + m Sweden. y = k x  + d Austria. y = p x  + q Netherlands.

Please, let me know if you are in a position to add to the above table (also if you can confirm or invalidate any part of it).  Thanks.

### Acknowledgements :

Information for Uruguay and Netherlands due to Julie Budnik (2005-12-19).

(Bob of Sacramento, CA. 2000-12-04)   Diamond Mark
What is the mark at  19" 3/16  on a tape measure?  It repeats itself.

The so-called  diamond mark  is actually positioned at exactly 8/5 of a foot  (that's exactly 1.6' or 19.2 inches, which is indeed pretty close to 19" 3/16 ).

The diamond marks are also called "black truss" markings, because they correspond to the truss layout which is used with 8-foot sheets of plywood (or other material), namely 5 trusses per sheet.

This is to be contrasted with "red stud" markings which appear every 16 inches by showing the corresponding inch number in red instead of black. The black markings and the red markings coincide at 8-foot intervals (96 inches).  That's to say:  5 black intervals or 6 red ones in an 8-foot width.

5/8 = 0.625 is a standard slope for a roof, which may thus be built by measuring horizontally as many diamonds as there are vertical feet.

The ratio 8/5 = 1.6 is very close to the Golden Ratio, which has been used in architecture since antiquity...  The golden ratio is the  aspect ratio  of a rectangle whose larger side is to the smaller side what the sum of the two sides is to the larger side. It is also equal to the diagonal of a regular pentagon of unit side. Its precise value is f = (1+Ö5)/2, which is about 1.618034.
den0eng3 (2002-06-28)
What is the largest Excel expression of at most 35 keystrokes?

Excel interprets something like 3^3^3 as (3^3)^3 = 27^3 = 19683.  (For some calculators, this expression means 3^(3^3) = 3^27 = 7625597484987.)

This idiosyncrasy of  Excel  makes the question interesting, because of the parentheses needed to make a  tower  of exponents.  (Otherwise, the answer would simply be 9^9^9^9^9^9^9^9^9^9^9^9^9^9^9^9^9E9.)

N1 9 99 9E9 has 10 digits. 9E99 has 100 digits. 9^9E9 has 8588182585 digits. 9^9E99 has over 8.588´1099 digits. 9^9E999 has over 8.588´10999 digits. 9^9E9999 has over 8.588´109999 digits. 9^(9^9E9) has over 0.95424 ´ N5 digits. 9^(9^9E99) has over 0.95424 ´ N6 digits. 9^(9^9E999) has over 0.95424 ´ N7 digits. 9^EXP(9^9E9) has over 0.95424 ´ exp(N5) digits. 9^EXP(9^9E99) has over 0.95424 ´ exp(N6) digits. 9^EXP(9^EXP(9^EXP(9^EXP(9^9E99)))) 9^EXP(9^EXP(9^EXP(9^EXP(9^9E999)))) 9^EXP(9^EXP(9^EXP(9^EXP(9^9E9999)))) 9^EXP(9^EXP(9^EXP(9^EXP(9^(9^9E9))))) 9^EXP(9^EXP(9^EXP(9^EXP(9^(9^9E99))))) There is a good reason to stop the table at this point, as explained below.

A nice function to generate large numbers is the factorial function.  However, its Excel name (FACT) is longer than that of the exponential function (EXP) which is thus allowed to win the day for N12, in spite of its less extreme growth.

What happens next is illustrated by the N13 case, where we had to choose the largest candidate among  9^EXP(9^9E99)9^FACT(9^9E9),  and possibly 9^(9^(9^9E9))...  Because the argument of EXP has one more keystroke, the corresponding expression is the largest by a wide margin.  The somewhat larger growth of FACT does not help much, in this case or in any subsequent one...

For 12 keystrokes or more, the largest expression is "9^EXP(...)" with an inner expression found 7 steps before in the table (7 fewer keystrokes).  This makes the table extremely easy to extend beyond the 12th entry, and we may quickly obtain the final answer to the original question (35 keystrokes):

### 9^EXP(9^EXP(9^EXP(9^EXP(9^9E999))))

Only functions with exponential growth and shorter names, like CH or SH, could possibly allow this record to be broken, but in Excel such potential candidates have longer names which rules them out (namely, COSH and SINH).

For a larger number of keystrokes, the following technique may or may not be acceptable (as it's a pure Excel idiosyncrasy, not shared by similar languages).  The basic idea is to use compact descriptions of extremely long text strings (representing syntactically correct numbers) using the function REPT(x,n), which returns n concatenated copies of whatever string is specified by x.  Such strings can be explicitly converted into syntactically acceptable numbers using the EVALUATE function (which works if macros are allowed).  Either one of the following related patterns will thus convert a numerical expression W of length n into an expression of length 2n+35 or 2n+36 representing a tower of W exponents.  Which pattern you use depends on the parity of the allowed number of keystrokes, whenever that number is 39 or more.  The method doesn't apply to less than 37 keystrokes and is inferior to the above for 37 or 38 keystrokes.

```EVALUATE(REPT("9^(",W)&9&REPT(")",W))
EVALUATE(REPT("9^(",W)&99&REPT(")",W))```

The operator "&" is used to concatenate strings.  Quotes around the inner "9" or "99" are not needed since integers are converted to strings whenever appropriate. Thanks to den0eng3 for suggesting the use of EVALUATE...

N39 EVALUATE(REPT("9^(",99)&9&REPT(")",99)) EVALUATE(REPT("9^(",99)&99&REPT(")",99)) EVALUATE(REPT("9^(",9E9)&9&REPT(")",9E9)) EVALUATE(REPT("9^(",9E9)&99&REPT(")",9E9)) EVALUATE(REPT("9^(",9E99)&9&REPT(")",9E99)) EVALUATE(REPT("9^(",9E99)&99&REPT(")",9E99)) EVALUATE(REPT("9^(",9^9E9)&9&REPT(")",9^9E9)) EVALUATE(REPT("9^(",9^9E9)&99&REPT(")",9^9E9)) EVALUATE(REPT("9^(", Nn )&9&REPT(")", Nn )) EVALUATE(REPT("9^(", Nn )&99&REPT(")", Nn ))

Finding what string of given length describes the largest number in a given formal language is a variant of what's known as the Busy Beaver Problem  (the name comes from the impression you get from watching small Turing machines generate large outputs).  For a general enough computer language (i.e., anything as powerful as a  Turing machineno  algorithm could possibly solve this problem!  A single  Excel  expression falls short of that intractable category, but a whole  Excel  spreadsheet would be in it...

What are the odds in favor of being dealt a given poker hand?

There are C(52,5) = 2598960 different poker hands and each of them is dealt with the same probability.  [See details elsewhere on this site.]

The probability of a given type of hands is thus the number of such hands divided by 2598960.  When the probability of something is the fraction P = x / (x + y) , its so-called odds are said to be either x to y in favor or y to x against, as shown in the table below, which assumes some familiarity with poker  (10 kinds of "straights" are normally allowed, see below or here).

TypeNumber of HandsProbabilityOdds in Favor
Royal FlushC(4,1) C(1,1) 1 / 6497401 to 649739
Straight FlushC(4,1) C(10-1,1) 36 3 / 2165803 to 216577
4 of a KindC(13,1) C(48,1) 624 1 / 41651 to 4164
Full House13 C(4,3) 12 C(4,2) 3744 6 / 41656 to 4159
FlushC(4,1) [C(13,5) - 10] 5108 1277/6497401277 to 648463
StraightC(10,1) (45-4) 10200 5 / 12745 to 1269
3 of a Kind13 C(4,3) C(12,2) 42 54912 88 / 416588 to 4077
Two PairsC(13,2) C(4,2)2 44 123552 198 / 4165198 to 3967
Pair13 C(4,2) C(12,3) 43 1098240 352 / 833352 to 481
High Card(C(13,5)-10) (45-4) 1302540 1277 / 25481277 to 1271
TOTAL C(52,5)2598960 11 to 0

In the last entry, "High Card" means a hand that's none of the above :  Two such hands would be compared highest card first to decide who wins.

Note that there are normally 10 different "heights" for a straight and that the ace (A) belongs to the lowest (A,2,3,4,5) and the highest (10,J,Q,K,A), which is traditionally called a Royal Flush if all cards belong to the same suit.  Should your own local rules disallow the tenth straight sequence (A,2,3,4,5), the tabulated counts for straights and/or flushes should be changed (and the "High Card" count should be modified as well), replacing 4, 36, 5108, 10200 and 1302540 respectively by 4, 32, 5112, 9180 and 1303560 = (C(13,5)-9)(45-4).

(A.C. of Houston, TX. 2001-02-12)
What are the numbers in reverse sequence on the verso of a book's title page, below the publication date?

The last of them indicates the number of the printing run for the copy you're holding: "10 9 8 7 6 5" means "fifth printing". There's also a similar sequence of double digits which indicates the date of printing: The sequence "02 01 00 99 98" means "98", which would most probably be 1998, since I do not think [I may be wrong on this] that the system was in force in 1898 or earlier. In practice, the system would remain unambiguous even in the distant future, since the latest date appearing elsewhere on the page can't possibly predate the actual year of printing by more than a century... Please, let me know if you have any information about approximately when this practice started. Thanks.

The reason for this strange convention is quite practical: It allows the same plates to be used for each printing; the last number is simply carved out as needed for a new printing, so that it no longer appears on the paper. This saves time and money with traditional printing.

(2002-07-13)
How many living species are there on Earth?

Approximately 1400 000 species have been recognized, but the total number of species is estimated to be at least 10 000 000.

However, for all we know, the actual number could be as high as 100 million.  There is currently no central database established by a recognized authority, although this may change with ongoing or future efforts, like the Species 2000 project, the ALL Species Foundation, or the Census of Marine Life (CoML).

Originally, CoML was an initiative of the  Consortium for Oceanographic Research and Education  (CORE)  before its 2007 merger with the  Joint Oceanographic Institutions  (JOI)  which resulted in a new entity dubbed  Consortium for Ocean Leadership  (COL, activated in 2008).

In his 1992 book The Diversity of Life (Harvard University Press), Edward O. Wilson quotes a total number of 1402 900 identified species.  This inventory includes 751 000 insects, 123 400 noninsect arthropods, 106 300 other invertebrates, and only 42 300 vertebrates (less than 10% of which are mammalian).  The remainder consists of 248 400 plants, 69 000 fungi, 57 700 protists (inluding 26 900 phototropic algae), and 4800 bacteria (the bacterial world is almost uncharted, see below).  Wilson himself believes the actual total number of species alive on Earth to be "somewhere between 10 and 100 million".  This seems to be the most often quoted range, although the Oxford specialist Robert M. May offers a much lower guess of 5 to 8 million.

Wilson estimates that about 27 000 species disappear each year (about 3 per hour), mostly because of the eradication of the rain forest.  This amounts to 1000 or 10 000 times the "natural" extinction rate prevalent in prehistoric times.  In his 1994 book Vital Dust (BasicBooks), 1974 Nobel laureate Christian de Duve quotes all of the above and calls this the biological equivalent of the burning of the library of Alexandria in 641.

Even if cataloguing them would essentially be an endless task, the number of bacterial species in a given sample can be estimated statistically by measuring only the total population and the number of individuals in the most prominent species.  Dr. Tom Curtis (and his coworkers at the University of Newcastle upon Tyne) did just that in a recent article of the Proceedings of the National Academy of Sciences:  A cubic centimeter of seawater typically holds about 160 species, and the entire ocean is expected to contain about 2000 000 distinct species of bacteria.  On the other hand, a gram of garden soil harbors around 6300 species, and a ton may contain about 4000 000 of them.

(2002-11-14)
What are the most primitive species still alive?

By studying genetic material at the molecular level (DNA), cladists are now able to obtain a fairly accurate picture of what the DNA of a group's common ancestor was like.  They can also determine what species is closest to that ancestor and is thus the most primitive of them, probably because it has been around the longest...  Examples includes:

(Cortney C. of Anacoco, LA. 2000-10-10)
How many dimes are in an ounce? How many pennies are in an ounce?
(C. S. of Rayne, LA. 2000-08-22)
When did the penny become a gram lighter?
(Anna of Rock Rapids, IA. 2000-10-11)
What is the volume of a penny?

I assume you're talking about US coins.

Pennies manufactured from 1793 to 1837 were pure copper. Before 1982, the penny was still almost a solid copper coin (95% copper, 5% zinc) and its nominal mass was set at 48 grains (about 3.11g). It was legally allowed to be as much as 2 grains above or below the nominal value, but practical tolerances were much tighter.

In the context of coins, the troy ounce of 480 grains is more appropriate than the common avoirdupois ounce of 437.5 grains. So your first answer is that it takes 10 (pre-1982) US pennies to make a troy ounce (and about 9.1146 of these to make an avoirdupois ounce).

Because newer pennies are about 20% lighter (nominally 2.5 g, as stated below), the average mass of a penny in a mix of new and old ones is roughly 10% less than a pre-1982 penny, and it's therefore close enough to 1/10 of an avoirdupois ounce (see next article). This approximation is used [without explanation] in the superbly crafted MegaPenny Project at kokogiak.com.

Since November 1982, the penny has been a copper-plated zinc coin (97.6% zinc, 2.4% copper) with a nominal mass of 2.5g [0.6 g lighter than before].

Some pennies manufactured in 1982 were reported to have a plating 3 times thinner (99.2% zinc, 0.8% copper). We are unable to confirm.

When the change from copper to zinc took place, the new pennies were engineered to have the same look and size as the old ones. With such a nominal mass, the volume of the new penny was only reduced by 0.84% (see computation below). Since the nominal diameter of the coin was held constant, this means its thickness went down by 0.84%.

Let's put ourselves in the shoes of whoever had to design the 1982 penny. To keep a constant size, the new penny should roughly be to the old one in terms of mass what zinc (7.1 g/cc) is to copper (8.9 g/cc) in terms of density. A more precise estimate may be obtained by looking up precise density values. We found copper listed at 8.960 g/cc at 20°C. Zinc is listed at 7.133 g/cc at 25°C. To get its density at 20°C, we use Zinc's coefficient of linear expansion (listed as 3.02´10-5/°C at 25°C) so its density at 20°C is about (1 + 4.53´10-4 ) larger, or about 7.136 g/cc. We may assume --although that's not quite true in practice-- that a gram of an alloy made from X grams of copper and (1-X) grams of zinc has the same volume as the total volume of the two component metals taken separately: X grams of pure copper (density d1) on one side and (1-X) grams of pure zinc (density d2) on the other. With this assumption, the density of such an alloy is: 1/(X/d1+(1-X)/d2). This makes the density of old pennies about 8.847 g/cc and that of new ones about 7.171 g/cc, for a ratio of about 81.06%. As the old penny weighs a nominal 48 grains (about 3.1103g), a new penny of the exact same size would weigh about 2.521 g. At a nominal mass of only 2.5 g, a new penny has therefore a volume which its about 0.84% smaller than the volume of an old penny...

The volume of a penny is very close to 0.35 cc (0.0214 cubic inches); slightly more for a copper penny, slightly less for a zinc penny: The volume of a copper penny is about 0.3516 cc. This is obtained as the ratio of the nominal mass of a "copper" penny (3.1103 g) to the approximate density (8.847 g/cc) of the 95% copper alloy it is made of. Similarly, the volume of the "zinc" penny is about 0.3486 cc, the ratio of the new nominal mass (2.5g) to the new density of 7.171 g/cc (that's 0.84% less than the volume of a copper penny).

Dimes are about 2.268 g each. The nominal mass is 35 grains (2.26796185 g). With dimes, quarters, or half-dollars (see below), \$20 worth of coins make an avoirdupois pound (7000 grains). There are 200 nominal dimes in 7000 grains.

Isotopic Pennies.

The 20% difference in mass between pre-1982 and post-1982 US pennies is used as the basis of a classroom activity (known as "Isotopic Pennies") which is meant to help chemistry students grasp how the average molar mass is related to the isotopic composition. For example, a single weighing of a stack of 10 pennies determines how many pre-1982 pennies are in it...

Thanks to Gene Nygaard of Crosby (ND) for pointing out the nominal masses of the new penny (2.5 g), the nickel (5 g) and other coins...  \$20 per pound is the current standard for dimes and quarters (since 1965) as well as half-dollars (since 1971).  The same was true of the so-called "Eisenhower dollars" minted between 1971 and 1978 (1½" or 38.1 mm in diameter, 350 grains or about 22.68 g in weight).
From 1873 to 1964, on the other hand, dimes, twenty-cent pieces, quarters and halves were \$40 per kg.  In other words, new coins are to the old ones what an avoirdupois pound (453.59237 g) is to a metric pound (500 g).  That is, the mass of new coins is nominally 90.718474% that of the old ones. Between 1965 and 1970, half-dollars were nominally 11.5g, which is about 1.4% higher than the current value of 175 grains (11.33980925 g).
For completeness, we should also state that current US dollar coins have a nominal mass of 125 grains (about 8.1 g) yielding 56 dollar coins to the pound...  The newer golden Sacagawea dollar (first minted in 2000) was designed to have the exact same weight, size, and electromagnetic signature as the Susan B. Anthony dollar, so both coins can be used concurrently in vending machines.

The size and mass of US coins has been legally enacted in 31 USC 5112 (Title 31, Section 5112 of the U.S. Code, also available from findlaw.com). As of 2001, paragraph (a) of Sec. 5112 still gives the specifications of the old pre-1982 penny; the leeway necessary for the "new" post-1982 penny is provided by paragraph (c).

US coin denominations withdrawn from circulation include the half-cent (1793-1857), 2¢ (1864-1873), 3¢ silver (1851-1873), 3¢ nickel (1865-1889), half-dime (1792-1873), and 20¢ piece (1875-1878).  There are arguments for and against the withdrawal of pennies (some polls show that 70% of Americans wish to keep them).

US gold coins have been minted in the following denominations:  \$1 (1849-1889), \$2½ "quarter eagle" (1796-1929), \$3 (1854-1889), \$5 "half eagle" (1795-1929), \$10 "eagle" (1795-1933), \$20 "double eagle" (1850-1933).
Current law would only allow the minting of \$5, \$10, \$25 and \$50 gold coins...

(Robert of Clifton, TX. 2000-11-11)     US Pennies by the pound.
How many pennies are in a pound?
[How many pennies per avoirdupois pound?  US pennies in 1 lb.]

Assuming you're talking about US coins, there's a big problem: In November 1982, the US penny became about 0.6 gram lighter. The older coin was 95% copper and 5% zinc, while the new one is essentially copper-plated zinc (97.6% zinc and only 2.4% copper). The nominal mass of a penny before 1982 was 48 grains (about 3.11 g). The size of a penny changed very little (-0.84%) in 1982 but, because zinc is lighter than copper, the new coin's nominal mass is 2.5 g.

The price of copper had risen to \$1.33 per pound in 1980, so pennies could not be minted for less than their monetary value.  Copper-plated coins postponed the crisis  (in 2000, it cost 0.81¢ to mint a penny).

Before 1982, there was about 146 pennies in a pound...  If all the pre-1982 pennies were out of circulation, there would be about 181 pennies to the pound.

Right now, a pound of pennies from the street will contain anywhere between 146 and 181 pennies, depending on the percentage of pre-1982 pennies in it.  According to the US Mint, the approximate life span of a coin is about 25 years.  If we take this number at face value, there remains in circulation today (November 2000; 18 years later) approximately exp(-18/25), or about 48.7% of the pennies that were in circulation in November 1982.

Assuming that the total number of pennies in circulation is the same today as it was in 1982 (which is probably not quite true), this would mean that a penny's mass in grams averages about 3.11(0.487)+2.5(1-0.487) which is very close to 2.8 g, so that there would be just about 162 pennies in a pound as of November 2000.  If there's already more than 160 pennies in a pound, the average penny is already slightly less than 1/10 of an avoirdupois ounce!

The above theoretical approach tells how the number of pennies in a pound varies with time, if we assume that the total number of pennies in circulation is held roughly constant...  However, it's all based on the "approximate life span" of 25 years quoted by the US Mint, which could be an overestimate for pennies.  If we are to believe some fundraisers, an average pound of pennies was already worth \$1.64 (164 pennies to the pound) as early as September 1995, only 13 years after the introduction of the new penny.  164 pennies in a pound (of 453.59237 g) corresponds to a proportion (x) of pre-1982 pennies which is such that:  164  =  453.59237 / (3.11034768 x + 2.5 (1-x))   This would mean that x was already as low as 43.55% after only 13 years or so, implying that the average number of years (T) that a penny lives is such that exp(-13/T) = 0.4355.  T would thus be about 15.64 years, about  2/3  of what the US Mint states for its other coins.
samgiordano (2003-05-04; e-mail)
How much money is in five gallons of nickels?
[How many pennies, dimes or quarters per gallon?]

We're talking about US coins (5 cents) and US gallons (namely Winchester gallons of exactly 231 cubic inches, which are very different from Imperial gallons)...

We'll consider nickels to be perfect cylinders.  Packing identical solids as densely as possible is a notoriously difficult problem (only recently solved for spheres).  For circular cylinders, we may guess that the solution involves optimal 2D layers (not necessarily aligned with each other) as illustrated above.  Alternately, unlayered stacks of cylinders arranged next to each other in this 2D hexagonal pattern fill 3D space with the same density.  Denser packings do not seem possible, although we lack a rigorous proof of this "obvious" fact.  This is how we may estimate the highest number of nickels per gallon in large containers...

The nominal diameter of a nickel is 0.835", or 21.209 mm (see 31 USC 5112).  The US Mint online specifications give the thickness of a nickel as 1.95 mm, but I did a quick reality check by measuring a stack of 20 nickels and found it to be almost exactly 37 mm, instead of the expected 39 mm!  Assuming a one-digit typo in the official site, we'll thus take the thickness of a nickel to be 1.85 mm.  The discrepancy is otherwise much too large to be attributed to normal wear...  [Similar thickness measurements for other types of circulated coins match the nominal data published online by the US Mint  almost perfectly.]

In the aforementioned packing(s), each coin  occupies  (without voids or overlaps) the volume of a regular hexagon of the same thickness circumscribed to it.  The top surface area is ½Ö3 times the square of the coin's diameter.  As there are exactly 25.4 mm to the inch, the above numbers make this volume in cubic inches equal to:

V   =   ½Ö3 (0.835)2 (1.85/25.4)   =   0.0439786196844...

Since there are exactly 231 cubic inches in a US gallon, this translates into 231/V, or about 5252.5523 nickels per gallon.  In 5 gallons, you'd have at most 26262 nickels, worth  \$1313.10  (possibly a little bit more if the walls of the container are shaped to fit the coins, but much less in a random packing).

For pennies (diameter: 0.75", thickness: 1.55 mm) the above computation would give a "V" of about 0.029727 cu in, or about 7771 pennies per gallon.

Similarly, a gallon would contain [at best] roughly 10500 dimes, 4200 quarters or 2100 half-dollars.  Any of these translates into \$1050 per gallon.  The result is the same for these 3 types of coins for a reason you have to figure out, but here's a clue:  Any mixture of these three types of coins represents the same amount of money, by weight.  Seasoned geometricians would remark that a given volume would be less valuable with several coin types instead of a single one.  All this, of course, is for the ideal densest possible packing...  In practice, YMWV.

 On 2007-09-11, Bob Bernstein  [ BongoJava | TuesdaysWithMax ]  wrote: I just counted my very, very full to the rim gallon of pennies.I only found  5612  pennies, instead of your estimate of  7771.

As advertised, the physical packing obtained by stuffing coins in a jar will be substantially less dense than the densest packing of cylinders described above.  It's not at all surprising to find  27%  fewer coins in such a jar than in an equivalent volume of neatly stacked coins.