lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



If then we connect a hole in the side of the tunnel with one end of 

 a liquid manometer, and leave the other end open to the room, the 

 gage reading is proportional to the difference in pressure or to 



Pr-P = p0-P=^-^- 



The reading of the manometer thus is a measure of the velocity. 

 Due to loss of head from friction in the mouth of the tunnel and 

 in the honevcomb, the relation 



is not strictly true. An unknown loss in friction would be repre- 

 sented by adding a term to indicate the friction head pressure. Then 



Pr = p +'''"'- +pf. 

 2^ 



The use of the side plate method ignores the effect of pf. A com- 

 parative test showed an error of 3 per cent when velocity was calcu- 

 lated from side plate readings. It is, therefore, necessary to calibrate 

 the side plate and its manometer against the standard Pitot tube and 

 its manometer. 



The side plate used (fig. i ) consists of a thin brass disk about 3 

 inches in diameter set flush in the wall of the tunnel. The disk is flat 

 and highly polished. Near its center, five holes 0.02 inch in diameter 

 arc drilled. These holes are connected with a brass tube soldered to 

 the back of the plate and projecting through the side of the channel. 

 Rubber tubing is used to transmit the static pressure from the small 

 holes to one end of a manometer. As explained above, the other end 

 of the manometer is open to the air in the room. 



The pressures transmitted by the side plate have been found to 

 respond very quickly to changes in velocity, and the method is even 

 more sensitive than the Pitot tube. Naturally its precision is no 

 better than that of the Pitot used for its calibration. 



The pressure difi^erence transmitted by the side plate is read on an 

 inclined alcohol manometer on the Krell principle. Both the side 

 plate and this alcohol manometer require calibration against a 

 standard. For convenience, the side plate and its manometer were 

 calibrated together against the standard Pitot tube and a Chattock 

 manometer. 



The standard National Physical Laboratory Pitot tube w^as 

 mounted in the center of the tunnel in the place where models are 

 tested. This tube was connected to the Chattock gage. The side 

 plate in the wall opposite the tube was then connected to the alcohol 



