Sphere and cylinder. I would have uploaded a model, but as new user i’m not allowed.
I would like to know the length of the intersection curve.
If somebody knows how to achieve, would be great to tell me.
thx
…
thx @Okidoki
I recognized the add-on.
As beginner i now need to find out how to use it 
As i understand, the add-on simply adds the length of straight lines between the nodes. So its accuracy depends on the number of nodes being used to get the model.
You simplt select the edges you want to measure and it “visualize” the notation of the single edges and sum up every selected ones in the given letters separately… (have to press ths “do-it” button 
)
Hello,
Another possibility:
In the “Preferences” activate the “Curve Tools” Add-on,
Do your Boolean operation,
Select your intersection (Edit mode), separate it,
Convert it to a curve, then transform it into a “Bezier” curve (Edit mode - Set Spline Type - Bézier)
Finally in the right side panel (N), under the “Edit” tab, open “Curve Info” click “Length” (it is displayed in the right box next to it) !
Good 
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thx @Okidoki and @JaAlVir657
I needed a lot of learning and trial and error. But finally i got something valuable:
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Didn’t try it in new blender versions…
sorry
Edit1: work in 4.2 also