You will need
- - ruler or measuring tape;
Measure the resistance of the wire. For that its ends connect the tester included in the mode of operation of the ohmmeter. The device screen will be an electric resistance wire in ohms or multiples of them are values, depending on instrument settings. The wire must be disconnected from the power source.
Calculate the resistor using tester, which operates in the mode ammeter and voltmeter. If the wire is a plot of the electric circuit, connect it to the power supply. To the ends of the wire in parallel connect the tester included in the operating mode of the voltmeter. Measure the voltage drop across the wire in volts.
Switch the tester in the mode of operation of ammeter and switch it into the circuit in series. Get the value of current in circuit in amperes. Using the ratio obtained from Ohm's law, find the electric resistance of the conductor. To do this, divide the value of the voltage U and the current I, R=U/I.
Example. The measurement showed that the voltage drop on the conductor 24 V, current is 1.2 A. Determine the resistance. Find the ratio of voltage to current R=24/1,2=20 Ohms.
Find the resistance of the wire, not connecting it to the power source. Find out what material made the wire. In the specialized table, find the resistivity of material in Ohm∙mm2/m.
Calculate the cross-section of the wire, if it is not specified initially. To do this, clean it from isolation if it is isolated, and measure the diameter of conductor in mm. Determine its radius by dividing the diameter by the number 2. Determine the cross section of the wireby multiplying the number π≈3,14 the square of the radius of the core.
With a ruler or tape measure measure the length of wire in meters. Calculate the resistance of the wireby multiplying the resistivity of the material ρ on the length of the conduit l. Divide the result by its cross section S, R=ρ∙l/S.
Example. Find the resistance of copper wire with a diameter of 0.4 mm with a length of 100 m. the resistivity of copper equal to 0,0175 Ohm∙mm2/m. the Radius of the wire is equal to 0.4/2=0.2 mm. cross-Section S=3,14∙0,22=0,1256 mm2. Calculate the resistance by the formula R=0,0175∙100/0,1256≈14 Ohms.