Compressed air is delivered through tube 1, partly to internally tapered glass indicator column 2 and partly through conical valve 3. After this the air passes through orifice 4 and chamber 6 to nozzle 5 of the measuring head or tracer. The head consists of stylus 7 mounted on flat steel reeds 8. The upper end of the stylus is a valve controlling the flow of air from nozzle 5. In operation, the tracer is placed with its fixed pins a on the surface whose roughness is to be measured and is moved along the surface at a definite speed. At this, stylus 7 oscillates vertically in accordance with the irregularities of the surface. This continuously varies the flow of air through orifice 4. For a high enough frequency of oscillations of the stylus and also for a properly chosen volume of chamber 6 and diameter of orifice 4, the indicator float in the flow gauge is damped and maintains a position corresponding to the average value of air flow. This is an indication of the average height of surface irregularities.
$3711$SHP,M$