Returns the thickness of the circular arc entity (DXF 39) in drawing units. Thickness is the extrusion length along the normal. A positive value defines the thickness to be drawn along the normal direction. A negative value defines the thickness to be drawn in the opposite direction from the normal. A zero value defines an arc without thickness.

Example of Working with the Circular Arc 

Specific Circular Arc Properties 

OdDbArc::setThickness() method