19. An aluminium wire 7.5 m long is connected in parallel with a copper wire 6 m long. When a current of 5 A is passed through the combination, it is found that the current in the aluminium wire is 3 A. The diameter of the aluminium wire is 1.0 mm. Determine the diameter of the copper wire. Resistivity of copper is 0.017 μm; that of aluminium is 0.028 μm. 20. The field winding of a d.c. motor is connected directly across a 440 V supply. When the winding is at the room temperature of 17 °C, the current is 2.3 A. After the machine has been running for some hours, the current has fallen to 1.9 A, the voltage remain- ing unaltered. Calculate the average temperature throughout the winding, assuming the temperature coefficient of resistance of copper to be 0.004 26/°C at 0 °C. 21. Define the term resistance-temperature coefficient. A conductor has a resistance of R, Q2 at 6, °C, and consists of copper with a resistance-temperature coefficient a referred to 0 °C. Find an expression for the resistance R₂ of the conductor at temperature 6, °C. The field coil of a motor has a resistance of 250 2 at 15 °C. By how much will the resistance increase if the motor attains an average temperature of 45 °C when running? Take a = 0.004 28/°C referred to 0°C. 22. Explain what is meant by the temperature coefficient of resistance of a material. A copper rod, 0.4 m long and 4.0 mm in diameter, has a resistance of 550 μ2 at 20 °C. Calculate the resistivity of copper at that temperature. If the rod is drawn out into a wire having a uniform diameter of 0.8 mm, calculate the resistance of the wire when its temperature is 60 °C. Assume the resistivity to be unchanged and the temperature coefficient of resist- ance of copper to be 0.004 26/°C. 23. A coil of insulated copper wire has a resistance of 150 2 at 20 °C. When the coil is connected across a 230 V supply, the current after several hours is 1.25 A. Calculate the average temperature throughout the coil, assuming the temperature coefficient of resistance of copper at 20 °C to be 0.0039/°C..