What model correctly predicts the sensitivity of a resisitive wire straing gage to applied strain?

This question is motivated by an an old pamphlet found in a used book store, "Theory Of Strain Gage Flight Test Instrumentation" by Eugene Frank, 27 May 1946.

The gage factor of a strain gage made from resistive wire is defined as
[itex] K = \frac{\triangle R/ R}{\triangle L/L} [/itex]
where R is the total resistance of the un-strained gage, L is the total length of un-strained "the specimen" being measured and the deltas are the respective changes in these quantities when a load is applied.

Frank says:

Frank's model assumes the change in resistance of the wire is entirely due to the change in the length and diameter of the wire as it is stretched.