Apparatus and methods for integrating transmission cable into the body of selected downhole tools, such as drilling jars, having variable or changing lengths. A wired downhole-drilling tool is disclosed in one embodiment of the invention as including a housing and a mandrel insertable into the housing. A coiled cable is enclosed within the housing and has a first end connected to the housing and a second end connected to the mandrel. The coiled cable is configured to stretch and shorten in accordance with axial movement between the housing and the mandrel. A clamp is used to fix the coiled cable with respect to the housing, the mandrel, or both, to accommodate a change of tension in the coiled cable.