4 1-20.   Because its resistance changes very little with temperature changes, what conductor is best suited for use in measuring instruments? 1.     Copper 2.     Aluminum 3.     Manganin 4.     Silver 1-21.   If a 75-ohm sample of copper wire at 0ºC is heated to 30ºC, what is the approximate total resistance? (The temperature coefficient of resistance of copper at 0ºC is 0.00427). 1.      0.32 ohm 2.      9.61 ohms 3.    65.39 ohms 4.    84.61 ohms 1-22.   What definition best describes an insulating material? 1.    A material that has a very high resistance 2.    A material that has a very low resistance 3.    A material that has a very high conductivity 4.    A material that has a very low dielectric strength 1-23.   The dielectric strength of an insulating material is a measurement of the material's ability to resist electrostatic stress caused by what factor? 1.    Resistance 2.    Current 3.    Voltage 4.    Chafing or friction 1-24.   Insulation resistance can best be defined as the ability of an insulating material to resist what action? 1.    Current leakage 2.    Electrostatic stress 3.    Breakdown by voltage 4.    External factors acting upon the conductor 1-25.   For a material to be a good insulator, what two properties are most important? 1.    High dielectric strength and low insulation resistance 2.    High dielectric strength and high insulation resistance 3.    Low dielectric strength and high insulation resistance 4.    Low dielectric strength and low insulation resistance 1-26.   When rubber is used as the insulating material over a copper conductor, why is a thin coating of tin used between the two materials? 1.    To decrease the electrostatic stress 2.    To increase the insulation resistance of the rubber 3.    To prevent a chemical action from taking place between the copper and rubber 4.    To reduce the amount of insulating material required. 1-27.   What is the NEC for a rubber heat- resistance compound? 1.    RHH 2.    RWH 3.    RTW 4.    RWT