Question 4
In activity 19.4 the wires used for cases 4, 5, 6 and 7 must all have equal cross-sectional areas. Why?
The right answer is the one marked above.
So a difference in reading cannot be caused by the thickness. Those cases are testing the material and the length. If the thickness varied too, a difference in the reading could be caused by any of three things and would prove nothing.
This is the rule behind every one of the three comparisons: change one factor, hold the others still.