Sentence:  A small <WSD>pressure</WSD> unbalance displaces the diaphragm and changes the capacitance between the diaphragm and an electrically insulated plate spaced 0.001 in. apart ( for * * f ) . 
Meaning of the word marked with <WSD> tags:  the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit)