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      Manufactured circuit components which contain definite amount of resistance are called resistors. In simplest terms,resistor resist ,or oppose,the flow of electrons through a circuit. Every material offers some resistance,or opposition, to the flow of electric current through it . Good conductors , such as copper , silver, and aluminum , offer very little resistance. Poor conductors , or insulator, such ac glass, wood, and paper, offer a high resistance to current flow.
      The size and type of material of the wires in an electric circuit are chosen so as to keep the electrical resistance as low as possible. In this way ,current can flow easily through the conductors,just as water flow through the pipe between two tanks. If the water remains constant, the flow of water in the pipe will depend on how far the valve is opened. The smaller the opening is, the greater the opposition to the flow, and the lower the rate of flow in gallons- per - second.
      The same is true in electrical circuits: the larger the diameter of the wires, the lower their electrical bresistance( opposition) to the flow of curren through them. In the water analogy, pipe friction oppses to the flow of water between the tank. This friction is similar to electric resistance. The resistance of the pipe to the flow of water through it depends on:
      the length of the pipe,
      the diameter of the pipe, and
      the nature of the inside walls( rough or smooth)

      Similarly , the electrical resistance of conductors depends on:
      the length of the wires,
      the diameter of the wires, and
      the material of the wire( copper,aluminum, etc.)
      Temperature also affects the resistance of electrical conductors to some extent. In most conductors ( copper,aluminum,iron, etc.) the resistance increases with temperature. Carbon is an exception. In carbon ,the resistance decreses as temperature increases. Certain alloy of metals (managing and constant) have resistance that does not change appreciably with temperature,
      the relative resistance of several conductors of the same length and cross section is given in the following list, with liver as a standard of 1.0 and remaining metal arranged in order of ascending resistance:
      Silver 1.0 Copper 1.08
      Gold 1.4 Aluminum 1.8
      Platinum 7.0 lead 13.5
      Resistance is measured in Ohms . One ohm is the resistance of a circuit or circuit element that permits a steady current of 1 mapere ( 1 coulomb per second) to flow when a steady EMF of 1 volt is applied of the circuit.

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