Answer
$0.5\;cm$.
Work Step by Step
The number of spaces for the diameter $=1$.
Let the actual measurement is $=x$.
$\Rightarrow \frac{scale \;measurement \;1}{actual\;measurement\;1}=\frac{scale \;measurement \;2 }{actual\;measurement\;2}$
$\Rightarrow \frac{1\;space}{0.5\;cm}=\frac{1\;space}{x}$
Cross multiply.
$\Rightarrow x=0.5\;cm$.
