A linkage mechanism provides for the extension and retraction of rooms in recreational vehicles while maximizing the space inside the vehicle. The absorbent element is fixedly connected to the chassis at a first location and is detachably connected to at least one of the absorbent element and the chassis at a second location. This layer can provide useful data about the condition and operation of the furnace. X and y deflection means are positioned adjacent each of the emitters for directing the charged particles from each of the emitters to at least one of the detectors to provide more signal output and a reduction in deflection accuracy.