Sets the thickness of the circular arc entity (DXF 39) in drawing units. Thickness is the extrusion length along the normal. A positive value extrudes along the normal. A negative value extrudes opposite from the normal. A zero value defines an arc without thickness. The initial value is zero by default. 

Example of Working with the Circular Arc 

Specific Circular Arc Properties 

OdDbArc::thickness() method

OdDbArc Methods