INGLESE TECNICO -CONDUCTORS AND INSULATOR

In electrical work a conductor is defined as a material which allows the free passage of an electric current along it, while  the term insulator  indicates a material which offers an extremely  high resistance  to the passage of an electric  current
There is no such thing as a perfect insulator for all substance conduct electricity to a Greater o lesser extent, but the difference in resistivity  between  conductors and insulator  is so enormous that there is no possible ambiguity in this classification
Thus slate , which  is a relatively poor insulator has nevertheless a resistivity  10/12 times higher than a high-resistance  alloy, which is a bad conductor  while the ratio between the resistivity  of the worst and best  conductors is much smaller
the resistivity of nichrome for istance is about seventy times the of copper
those materials which fall in between conductor and insulator  are classed as semi-conductors (germanium
The resistivity of any material is a constant for any specified temperature m but  varies with this 
The resistance of copper and most othe r conductor  increases with temperature
the resistance of  constantan and other usful alloy  is pratically constant within wid limits of temperature  while resistance of carbon of electrolytes and  of most  dielectrics decreases as  the  temperature rises
Because of this decrease in resistance which occurs in insulating materials a limit in the rise in temperature is imposed in their applications, otherwise the insulation would break down to cause a leakage current or even a short circuit
conductor and insulators  used in electrical work are extremely  veried and of a most diverse nature 
Because no single material can be used too extensively both for costs and availability  different materials are usually combined  to fhve the required  properties  of mechanical strength adaptability and  reliability