A developing unit includes a developing roller including a roller portion. Concentric tubular members are disclosed, wherein one tubular member, such as the drill string, is utilized to inject the drilling fluid into the wellbore. In another embodiment, the apparatus includes a substrate having a set of X electrodes, and a set of Y electrical leads, each connected to one or more electrodes, wherein X is greater than Y. If the DC offset mode is selected and a determination is made to perform DC offset mode, the blocking artifacts in a smooth region are removed in the DC offset mode. The inner shell has a contact surface in its upper portion and is arranged to be pressed towards the lower portion of the crushing head by a pressing force which is obtained by tightening at least two threaded bolts, which are arranged in the upper portion of the crushing head such that their longitudinal direction is substantially parallel to the symmetry axis of the crushing head.