A detector with a telescoping support tube wherein a measuring probe is arranged at a front of a forward tube element and an electronics unit with a battery compartment, a handle element and axially offset rearward therefrom a U-shaped arm shell with two arm support bars are arranged at a rear tube element, wherein the arm support bars are pivotable relative to the rear tube element. The arm shell is axially moveable relative to the handle element and includes two arm shell elements which are laterally arranged at a support slide and pivotably moveable in a transversal direction of the support tube. The support slide is supported axially moveable by a support rail. According to the invention it is provided that the support slide is fixateable in a disengageable manner at the support rail at various locations through the arm shell elements.