The origin of the word dollar comes from the coins that Spanish emperor Carlos I ordered to coin at the beginnings of the XVIth century. They were of silver, (read Webster's wu for more precise information) and they had the same value as the German thaler.

A century later, when the spanish thalers began to circulate in North America, they were called spanish dollars, as someone wrote "dollars" instead of "thalers".

When the U.S.A. became a nation, the dollar acquired the category of national currency. In those coins were engraved the Columns of Hercules, the symbol for the Spanish Empire, that ultimately were converted into the sign that epitomizes dollars and money everywhere:


The 'dollar' is used by many countries besides the United States of America -- though the US is probably the "most known" user of it. Here is the short list of countries that use 'dollars' and what artwork appears on the runs of their bills (coins not included).

  • Australia
    Facing side features notable Australians. Reverse features native artwork. Adopted in 1966. Multicolored.

  • Canada
    Facing features pictures of notable Canadians and Queen Elizabeth II along with reverse of native avian species. Multicolored.

  • New Zealand
    Facing features 'the Queen' and native fauna on reverse. Adopted in 1967. Multicolored.

  • United States of America
    Facing features pictures of Presidents and famous US Landmarks on revverse. Green and Black.

  • Hong Kong
    Famous Hong Kong landmarks with notable historic figures on reverse. Multicolored.

  • Singapore
    Features local avian species. Multicolored.

  • Taiwan
    Some features notable historic figures and prominent structures on reverse, while others feature scenes with Chinese people and avian species on reverse. Multicolored.

  • Bermuda
    Queen Elizabeth II with avian species or maritime scenes on reverse. Multicolored

  • The Bahamas
    Queen Eliabeth II with avian species on reverse. Multicolored.

A unit commonly used at a nuclear reactor to indicate the reactivity of something in the core, in a relationship useful to changes in power of the reactor. The dollar ($) is equal to the reactivity of the object (ρ, expressed in Δk / keff) divided by the proportion of delayed neutrons that cause fissioneff). A cent is also used to represent 0.01 dollars. In order to explain what these terms mean, here is a basic lesson in nuclear physics in relation to reactivity.

The first value, Δk/keff, is the departure of a nuclear reactor from criticality. The denominator, keff, is a multiplication factor that tells you how fast your pool of neutrons is growing. keff is the ratio of neutrons in your reactor right at this instant to the number you had one generation ago, before those neutrons smashed into your fuel and created new neutrons. For uranium 235 (or 235U), for example, every time an atom is activated by a neutron it spits out two neutrons, and a generation is the time it takes for all of the 'splitting' neutrons to hit uranium and create a new pool of 'split' neutrons. Of course, this is theory rather than practice; neutrons split constantly, so a generation is a type of approximation rather than a physically meaningful time. As well, not every neutron will be able to fission fuel; some escape, some are absorbed by other things in the reactor, and some are going the wrong speed(See: fast neutrons vs. thermal neutrons). In order to determine keff for a particular reactor, a number of different factors are taken into account, and not all of the values can be determined theoretically.

In the end, the important thing about keff is that it tells you how fast the neutron supply is growing or shrinking, or if the reactor is sustaining its neutrons by replacing them at the same rate that they are being absorbed or escaping. If keff is less than one, there are less neutrons every generation and the reactor is subcritical; if it equals 1, the reactor is critical and not changing, and if it is greater than one, the reactor is supercritical and power is increasing.

Δk is the distance that keff is from critical, or keff - 1. Mathematically, this means that the reactivity of a substance is 1 - 1/keff. An object can have positive or negative reactivity depending on what it does to a reactor when it is inserted into the core; adding something like fuel, which has a positive reactivity, increases the number of neutrons each generation, while borated carbon, a compound used in control rods with negative reactivity, will decrease the number of neutrons each generation.

βeff, or beta effective is defined as the fraction of delayed neutrons that cause fission. A delayed neutron is one that is created from a fission product, not from a fission reaction. For example, when uranium fissions, one of its products is bromine-87, which decays into krypton-87, and krypton decays further into krypton-86 and spits out a neutron. This neutron is a delayed neutron, and while there aren't a lot of them in comparison to the rest of the neutrons from direct fission(also called prompt neutrons), they do help control the time of a neutron generation, and thus the speed a reactor goes to power.

In order to take into account all of these factors, the dollar as a unit of reactivity divides the orignal reactivity by the βeff, getting a unit that takes into account the effect of delayed neutrons on the change in reactivity of the core. Like keff, βeff must be calculated for a specific nuclear reactor. This unit is especially useful when discussing types of criticality and power increases; the higher dollar value the reactor core has, the faster its power is increasing, and the more the power increase is due to prompt neutrons rather than delayed neutrons.

So why a dollar? When the Manhattan Project was first underway, a lot of code words were created to talk about nuclear physics. Dollars and cents were a handy decimal system already in place, with a normal enough usage elsewhere to make talking about "fifty cents worth of fuel" less suspicious. The usage has become so regular that when operators talk about the reactivity of something in the core, they talk about its "worth."

Dol"lar (?), n. [D. daalder, LG. dahler, G. thaler, an abbreviation of Joachimsthaler, i. e., a piece of money first coined, about the year 1518, in the valley (G. thal) of St. Joachim, in Bohemia. See Dale.]

1. (a)

A silver coin of the United States containing 371.25 grains of silver and 41.25 grains of alloy, that is, having a total weight of 412.5 grains.


A gold coin of the United States containing 23.22 grains of gold and 2.58 grains of alloy, that is, having a total weight of 25.8 grains, nine-tenths fine. It is no longer coined.

⇒ Previous to 1837 the silver dollar had a larger amount of alloy, but only the same amount of silver as now, the total weight being 416 grains. The gold dollar as a distinct coin was first made in 1849. The eagles, half eagles, and quarter eagles coined before 1834 contained 24.75 grains of gold and 2.25 grains of alloy for each dollar.


A coin of the same general weight and value, though differing slightly in different countries, current in Mexico, Canada, parts of South America, also in Spain, and several other European countries.


The value of a dollar; the unit commonly employed in the United States in reckoning money values.

Chop dollar. See under 9th Chop. -- Dollar fish Zool., a fish of the United States coast (Stromateus triacanthus), having a flat, roundish form and a bright silvery luster; -- called also butterfish, and Lafayette. See Butterfish. -- Trade dollar, a silver coin formerly made at the United States mint, intended for export, and not legal tender at home. It contained 378 grains of silver and 42 grains of alloy. <-- dollar bill. A paper note printed by the Treasury, or by on of the Federal Reserve Banks under authority of the treasury, having the value of one dollar. Five dollar bill, ten dollar bill, etc. Notes with the value of five, ten, etc. dollars. See dolar bill. Prior to 1964 such notes could be redemed for the equivalent dollar value of silver coins, but in that year the backing of the currency with silver was discontinued. Such notes not convertible into precious metals at a fixed rate are called "fiat money", receiving their value solely from the good faith of the issuing government. -->


© Webster 1913.

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