Resistance of a composite conducting wire

[Linus Pauling]

1. An 80-cm-long wire is made by welding a 5.50 mm-diameter, 20-cm-long copper wire to a 5.50 mm-diameter, 60-cm-long iron wire. What is the resistance of the composite wire?
2. p = resistivity, R = pL/A 3. r = 0.001375 m goes into A = pi*r². I used the above equation for R separately for the length of the wire for each metal, then added them together. I figured they would just add linearly like resistors in series.

My calculation came out to be 9.80*10^-3 ohm + 5.72*10^-4ohm = 0.0104 ohm = 10.4 milliohm

 

[willem2]

Isn't r equal to d/2 ?

The method you described is OK, but I couldn't get the same result. Could you give
a more detailed calculation?