torsion balance, instrument used to measure small forces. It is based on the principle that a wire or thread resists twisting with a force that is proportional to the stress. The torsion balance consists essentially of a wire or thread attached at one end and arranged in such a way that a force applied at the other, or free, end tends to twist it out of shape. The force is measured by the extent to which the wire or thread is so twisted. Torsion balances are used to measure small electric, magnetic, and gravitational forces. One type is used to measure small weights. The invention of the torsion balance is commonly credited to the English geologist John Michell, who made his instrument c.1750, and to the French physicist Charles A. de Coulomb, who independently devised such a balance c.1777.

balance of trade, relation between the merchandise exports and imports of a country. The concept first became important in the 16th and 17th cent. with the growth of mercantilism. Mercantilist theorists believed that a country should have an excess of exports over imports (i.e., a favorable balance of trade) to bring money, which they confused with wealth, into the country. They urged legislation to restrict the use of foreign goods, encourage exports, and forbid the export of bullion. The importance of a favorable balance of trade remained unchallenged until David Hume, Adam Smith, David Ricardo, and John Stuart Mill concerned themselves with theories on the adjustment of balance of trade. The classical theory of the mechanism is that a country whose exports fall short of its imports must export part of its stock of gold, thereby affecting its price structure and its ability to compete on the world market. Today the balance of trade is regarded as only one of several elements that make up the balance of payments of a nation; the U.S. Dept. of Commerce issues reports on the current status of the balance of trade in goods and services on a monthly basis. Since the 1980s the value of U.S. imports has greatly exceeded exports, resulting in large trade deficits that complicated U.S. relations with its trading partners, particularly Japan, China, and the United States' partners in the North American Free Trade Agreement, Canada and Mexico.

balance of power, system of international relations in which nations seek to maintain an approximate equilibrium of power among many rivals, thus preventing the preponderance of any one state. Crucial to the system is a willingness on the part of individual national governments to change alliances as the situation demands in order to maintain the balance. Thucydides' description of Greece in the 5th cent. B.C. and Guicciardini's description of 15th-century Italy are early illustrations. Its modern development began in the mid-17th cent., when it was directed against the France of Louis XIV. Balance of power was the stated British objective for much of the 18th and 19th cent., and it characterized the European international system, for example, from 1815-1914. After World War I the balance of power system was attacked by proponents of cooperation and a community of power. International relations were changed radically after World War II by the predominance of two superpowers, the United States and the Soviet Union, with major ideological differences between them. After the 1960s, with the emergence of China and the Third World, a revived Europe and Japan, it reemerged as a component of international relations. With the collapse of the USSR in 1991, the United States, as the sole remaining superpower, has been dominant militarily and, to a lesser degree, economically.

balance of payments, balance between all payments out of a country within a given period and all payments into the country, an outgrowth of the mercantilist theory of balance of trade. Balance of payments includes all payments between a country and its trading partners and is made up of the balance of trade, private foreign loans and their interest, loans and grants by governments or international organizations, and movements of gold (capital account). A chronically unfavorable balance of payments, when debits exceed credits, may affect the stability of the nation's currency, particularly where exchange rates are no longer fixed. After World War II the International Monetary Fund was established to handle problems relating to the balance of payments and foreign exchange.

Since the late 1950s the United States has generally experienced an unfavorable balance of payments because of large-scale foreign aid, sizable U.S. investment in Europe, and major U.S. military investments abroad. In the early 1970s the United States, in an effort to create a more favorable balance of payments, announced (1971, 1973) a devaluation of the U.S. dollar. However, the increase in the cost of petroleum from the Arab states (1973-74) had a negative effect on the balance of payments in the United States and most countries in Western Europe. In addition, tight money policies and high deficits adversely affected the savings rate in the United States in the 1980s and caused the balance of payments to decline even further. As a result, the United States looked to foreign borrowing to fill the gap, but the interest payments only increased the shortfall in the balance of payments. In the late 1990s and 2000s the U.S. balance of payments reached record negative levels.

balance, instrument used in laboratories and pharmacies to measure the mass or weight of a body. A balance functions by measuring the force of gravity that the earth exerts on an object, i.e., its weight. Since the mass of an object is directly proportional to its weight, a balance can also be used to measure mass.

Types of Balances

The Equal-Arm Balance

The simplest type of balance, the equal-arm, or beam, balance, is an application of a lever. A uniform bar, the beam, is suspended at its exact center on a knife-edge set at right angles to it. The point of support is called the fulcrum. Two pans of equal weight are suspended from the beam, one at each end, at points equidistant from the fulcrum. Since the center of gravity of a uniform bar is at its midpoint, the beam supporting the pans will be in equilibrium, i.e., will balance on the knife-edge. A long pointer attached at right angles to the beam at the fulcrum indicates zero on a scale when the beam is at rest parallel to a level surface. It shows also the extent of swing of the beam on one side or the other, acting somewhat as a pendulum, when the beam is coming to rest. The object to be weighed is placed on one pan, and standard weights are added to the other until the balance of the beam is established again. The unknown weight can then be determined by adding up the standard weights in the pan.

The platform balance is a form of equal-arm balance in which two flat platforms are attached to the top side of the beam, one at each end. Such a balance has a rider, or weight, mounted on a bar that has a calibrated scale, is parallel to the beam, and connects the supports of the two platforms. This rider is moved along the bar, its edge marking decimal fractions of the unit weight.

The Unequal-Arm Balance

On the unequal-arm balance the beam is suspended at a point a very short distance from one of its ends. The object to be weighed is placed on this end, and a small known weight is moved out along the longer arm until balance is obtained. The unknown weight is then determined by using a formula involving the known weight and the distance of each weight from the fulcrum. One example of this type of balance is the steelyard, an ancient device still used in underdeveloped nations because of its portability and low cost; since the difference in length of the arms can multiply the effect of the smaller weight by a factor of 100 or more, a small steelyard hung from a tree can be used, for example, to weigh a side of beef.

The Spring Balance

A spring balance consists of a coiled spring fixed to a support at one end, with a hook at the other to which the body to be weighed is applied. Within the spring's limit of elasticity, the distance through which it is stretched is directly proportional to the weight of the applied body. A pointer and graduated scale attached to the spring convert this distance into a weight reading.

Accuracy of Balances

Although extremely accurate results can be obtained in measuring the weights of minute objects, it is physically impossible to construct any balance perfect enough to yield absolutely accurate determinations. For example, the analytical balance, a type of equal-arm balance, is used for delicate weighing in quantitative chemical analysis and in preparing pharmaceutical prescriptions; it must be kept in a glass case, since its accuracy is easily affected by dust and moisture. A spring balance does not retain its accuracy permanently, for no matter how carefully it is handled, the spring very gradually uncoils even though its limit of elasticity has not been exceeded.

For ordinary purposes the errors are so small that they are considered insignificant, but in chemical analysis it has been necessary to develop methods by which they can be further minimized. A so-called torsion balance, which depends on the twisting of a wire or thread, is employed for weighing, but the term is commonly used to indicate a device for measuring minute electrical and magnetic forces.

See scale.

Balance, The, English name for Libra, a constellation; also called The Scales.

Difference in value over a period of time between a nation's imports and exports of goods and services. The balance of trade is part of a larger economic unit, the balance of payments, which includes all economic transactions between residents of one country and those of other countries. If a nation's exports exceed its imports, the nation has a favourable balance of trade, or a trade surplus. If imports exceed exports, an unfavourable balance of trade, or a trade deficit, exists. Under mercantilism a favourable balance of trade was an absolute necessity, but in classical economics it was more important for a nation to utilize its economic resources fully than to build a trade surplus. The idea of the undesirability of trade deficits persisted, however, and arguments against deficits are often advanced by advocates of protectionism.

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Device used to measure the gravitational acceleration at the Earth's surface. It consists essentially of two small masses at different elevations that are supported at opposite ends of a beam. The latter is suspended from a wire that twists because the masses are affected differently by the force of gravity. When the wire is twisted, an optical system indicates the angle of deflection, and the torque, or twisting force, can be calculated. The torque is correlated with the gravitational force at the point of observation. Torsion balance may also refer to a device used in weighing, a type of equal-arm balance.

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Weighing device that uses the relation between the applied load and the deformation of a spring. This relationship is usually linear; that is, if the load is doubled, the deformation is doubled. Spring balances are widely used commercially. Those with high load capacities are frequently suspended from crane hooks and are known as crane scales.

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In international relations, an equilibrium of power sufficient to discourage or prevent one nation or party from imposing its will on or interfering with the interests of another. The term came into use at the end of the Napoleonic Wars to denote the power relationships in the European state system. Until World War I, Britain played the role of balancer in a number of shifting alliances. After World War II, a Northern Hemisphere balance of power pitted the U.S. and its allies (see NATO) against the Soviet Union and its satellites (see Warsaw Pact) in a bipolar balance of power backed by the threat of nuclear war. China's defection from the Soviet camp to a nonaligned but covertly anti-Soviet stance produced a third node of power. With the Soviet Union's collapse (1991), the U.S. and its NATO allies were recognized universally as the world's paramount military power.

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Systematic record of all economic transactions during a given period between residents (including the government) of one country and residents (including the governments) of other countries. The transactions are presented in the form of double-entry bookkeeping. The U.S. balance of payments, for example, records the various ways in which dollars are made available to foreigners through U.S. imports, U.S. tourist spending abroad, foreign lending, and so on. These expenditures are shown on the debit side of the balance. The credit side shows the various uses to which foreigners put their dollars, including paying for U.S. exports, servicing debts to the U.S., and the like. Foreign countries may acquire more dollars than they need to spend on U.S. goods and services and may hold the surplus or purchase gold or securities; or they may have fewer dollars than they need to purchase U.S. goods and services, and may acquire additional dollars by transferring gold, selling holdings in the U.S., and so on. Certain forms of transferring funds (e.g., large outflows of gold) are less desirable as a way of settling foreign debts than others (e.g., transfers of currency acquired through international trade). The International Monetary Fund helps address problems relating to balance of payments. Seealso balance of trade.

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Instrument for comparing the weights of two bodies, usually for scientific purposes, to determine the difference in mass. The equal-arm balance dates back to the ancient Egyptians, possibly as early as 5000 BC. By the early 20th century, it had been developed into an exquisitely precise measuring device. Electronic balances today depend on electrical compensation rather than mechanical deflection. The ultramicrobalance is any weighing device that serves to determine the weight of even smaller samples than can be weighed with the microbalance (which can weigh samples as small as a few milligrams), that is, total amounts as small as a few micrograms.

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