counductor and inslulator
a counductor has very low resistance.the best 1-silver,copper,alumnuim,copper is used becuase cheaper.
the cross section area of round wire is measured in circular mil is 0.001 in. teh area in circular mil equals the diameter in mils squared.
teh resistance of conductor can be found by R=P l/a where p is specific resistance.l is the langth.a is the cross section area .wire resistance increase directly with l.but decrease with a.
the voltage drop across a closed switch in a series circuit is zero volts.when open.,the switch has the applied voltage across it.
a fuse circuit components and wiring against overload;excessive current melts the fuse.
ionization in liquid and gases prouduces atoms that are not electrically neutral.these are ions.negative ions has an excess of electrons;positive ions have a deficiency of liquids and gase,electric current is a result of the movement of ions.
the resistance of pure metals increase with temperature.for semiconductors and liquid electrolytes,reesistnance decrease at higher tempratures.

an insulator has very high resistance.common insulating materails air,vacuum,rubber paper,glass,porcelain,shellac,plastics.
superconductors have pratically no resistance.
common circuit troubles are open in wire conductors;dirty contacts in switches;and dirt,oxides,and corrosion on connectors and terminals.