) are joined by a straight line, the chord, the deviation of the chord from the circle at the segment's midpoint is called the sagitta f:
For small angles, or 
, one obtains
If a curvature 
is measured by measuring f (e.g. for a particle track in a magnetic field), then
where s = projected length. By 
with P =
projected momentum, H= magnetic flux density and e= elementary charge 0.2998 GeV/ c T-1
m-1] one obtains
