NO. 4 



WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 



13 



Before a measurement is taken, the bulbs A and C are opened to 

 the air of the room and the frame tilted by moving the micrometer 

 until the top of the bubble B is brought tangent to the horizontal cross- 

 wire of the microscope. This is the zero reading. The bulbs A and 

 C are then connected to the two parts of a Pitot tube and the frame 

 tilted until the bubble is again on the cross-wire. The amount of tilt 

 is then read on the micrometer. 



Fig. 2. — Chattock micromanometer. 



Naturally too wide an excursion of the bubble will result in its 

 rupture. The loss of a bubble transfers a drop of water from A to B, 

 and hence a new zero reading must be found by balancing up again. 

 To avoid sudden change of pressure and breaking of the bubble, a stop 

 cock at E is fitted. This cock can be closed to make the instrument 

 portable, and in taking a reading an approximate balance is made 

 with E partly opened. The cock is then opened full. 



The gage is filled with a solution of salt and water of s. g. 1.06. 

 The addition of a little salt keeps the castor oil from growing cloudy. 



Two gages were constructed, one by a skilled glass blower and 

 the other by a student, with a view to determining the efifect of 

 workmanship and dimensions. The frame and stand were made 



