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V. § 58.] REFLEXION AT AN OPEN ORIFICE. 121
pipe, the rarefaction just within its mouth cannot become great, since the air is free to enter from all sides. The particles behind, which in the case of a continuous tube would move in the direction of the arrow and diminish the rarefaction, will therefore be sooner stopped: the pressure and density will be greater than at the corresponding place and time in the continuous tube : and thus a pulse of condensation will be produced and transmitted back along the pipe.
To look at the matter from a somewhat different point of view, the motion of the air in the open pipe will be practically the same as it would have been if the pipe had formed part of a continuous tube, but air had been removed at the point corresponding to the opening whenever a pulse of condensation arrived there and air had been injected at the same point whenever a pulse of rarefaction reached it.
By the above reasoning1 which the student should carefully compare with that of § 21, it is clear that reflexion takes place at an open as well as at a closed end of a pipe: with this important difference, however, that in the case of the open orifice condensation is turned into rarefaction and rarefaction into con-
1 I am indebted for this popular explanation of reflexion at the mouth of an open pipe to Mr Coutts Trotter, Senior Fellow of Trinity College, Cambridge.