Fundamental Electrical and Electronic Principles, Third Edition

24 Fundamental Electrical and Electronic PrinciplesWorked Example 1.19QA coil of wire has a resistance value of 350 Ω when its temperature is 0°C. Given that the temperaturecoefficient of resistance of the wire is 4.26 10 3 /°C referred to 0°C, calculate its resistance at atemperature of 60°C.AR 0 350 Ω ; α 4.26 10 3 /°C; u 1 60°CR1 R0( 1αθ 1)ohm; where R1is the resistance at 60C 350 { 1( 426 . 10360)}350 { 10. 22556}3501.2556so, R1 439.6Ω AnsWorked Example 1.20Q A carbon resistor has a resistance value of 120 Ω at a room temperature of 16°C. When it is connectedas part of a circuit, with current flowing through it, its temperature rises to 32°C. If the temperaturecoefficient of resistance of carbon is 0.000 48/°C referred to 0°C, calculate its resistance under theseoperating conditions.Au 1 16°C; u 2 32°C; R 1 12 0 Ω ; α 0.000 48/°CRR1 αθ1αθ1 12 2120 1 0 000 48 16 ( . )R 1( 0. 000 4832)21201.0078R2120R21.0078so, R 119.1 Ω Ans2Use of meters The measurement of electrical quantities is anessential part of engineering, so you need to be proficient in the useof the various types of measuring instrument. In this chapter we willconsider only the use of the basic current and voltage measuringinstruments, namely the ammeter and voltmeter respectively.An ammeter is a current measuring instrument. It has to be connectedinto the circuit in such a way that the current to be measured is forced toflow through it. If you need to measure the current flowing in a sectionof a circuit that is already connected together, you will need to ‘ break ’the circuit at the appropriate point and connect the ammeter in the‘ break ’ . If you are connecting a circuit (as you will frequently have todo when carrying out practical assignments), then insert the ammeter asthe circuit connections are being made. Most ammeters will have theirterminals colour coded: red for the positive and black for the negative.