RESISTANCE

in electricity, property of an electric circuit or part of a circuit that transforms electric energy into heat energy in opposing electric current. Resistance involves collisions of the current-carrying charged particles with fixed particles that make up the structure of the conductors. Resistance is often considered as localized in such devices as lamps, heaters, and resistors, in which it predominates, although it is characteristic of every part of a circuit, including connecting wires and electric transmission lines. The dissipation of electric energy in the form of heat, even though small, affects the amount of electromotive force, or driving voltage, required to produce a given current through the circuit. In fact, the electromotive force V (measured in volts) across a circuit divided by the current I (amperes) through that circuit defines quantitatively the amount of electrical resistance R. Precisely, R = V/I. Thus, if a 12-volt battery steadily drives a 2-ampere current through a length of wire, the wire has a resistance of 6 volts per ampere, or 6 ohms. Ohm is the common unit of electrical resistance, equivalent to one volt per ampere and represented by the capital Greek letter omega, W. The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area. Resistance also depends on the material of the conductor. See resistivity. The resistance of a conductor, or circuit element, generally increases with increasing temperature. When cooled to extremely low temperatures, some conductors have zero resistance. Currents continue to flow in these substances, called superconductors, after removal of the applied electromotive force. The reciprocal of the resistance, 1/R, is called the conductance and is expressed in units of reciprocal ohm, called mho. also called Underground, in European history, any of various secret and clandestine groups that sprang up throughout German-occupied Europe during World War II to oppose Nazi rule. The exact number of those who took part is unknown, but they included civilians who worked secretly against the occupation as well as armed bands of partisans or guerrilla fighters. Their activities ranged from publishing clandestine newspapers and assisting the escape of Jews and Allied airmen shot down over enemy territory to committing acts of sabotage, ambushing German patrols, and conveying intelligence information to the Allies. The resistance was by no means a unified movement. Rival organizations were formed, and in several countries deep divisions existed between communist and noncommunist groups. Initially, the communists took a pacifist line, but, after Germany's invasion of the Soviet Union in June 1941, they joined the underground and in some areas became dominant in it. In Yugoslavia the Serbian nationalist Chetniks under Dragoljub Mihailovic and the communist Partisans under Josip Broz Tito fought each other as well as the Germans, and the two major Greek movements, one nationalist and one communist, were unable to cooperate militarily against the Germans. A similar division emerged in Poland, where the Soviet Union backed the communist resistance movement and allowed the Polish nationalist underground, the Home Army, to be destroyed by the Germans in the Warsaw Uprising of autumn 1944. In the Ukraine, where the Germans were at first welcomed as liberators, the Nazi treatment of the Slavic peoples as inferior races provoked a national resistance movement that fought not only the Germans but also the partisans organized by the Soviets to harass the long German supply lines to the Eastern Front. In Belgium a strong communist-dominated resistance movement coexisted with a resistance group constituted by former army officers. The main Norwegian and Dutch organizations, on the other hand, were closely linked with the royal governments-in-exile. The Germans' dismissal of the legal Danish government in 1943 gave rise to a unified council of resistance groups that was able to mount considerable interference with the retreat of German divisions from Norway the following winter. Communists dominated the resistance movement in northern (occupied) France, although both there and in southern France (ruled by the puppet Vichy regime) other resistance groups were formed by former army officers, socialists, labour leaders, intellectuals, and others. In 1943 the clandestine National Council of the Resistance (Conseil National de la Rsistance) was established as the central organ of coordination among all French groups. Early the following year, various belligerent forces known as maquis (named from the underbrush, or maquis, that served as their cover) were formally merged into the French Forces of the Interior (Forces Franaises de l'Intrieur ). Many of the resistance groups were in contact with the British Special Operations Executive, which was in charge of aiding and coordinating subversive activities in Europe; and the British, Americans, and Soviets supported guerrilla bands in Axis-dominated territories by providing arms and air-dropping supplies. After the Allied landing in France on June 6, 1944, the FFI undertook military operations in support of the invasion, and it participated in the August uprising that helped liberate Paris. Resistance forces in other northern European countries also undertook military actions to assist the Allied forces.