12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 02 



ings will gladly be supplied to anyone contemplating the construction 

 of such a gage. 



The Chattock micromanometer was devised by Professor A. P. 

 Chattock and Mr. J. D. Fry for the precise measurement of very 

 small pressures. The gage is described by Dr. T. E. Stanton in the 

 Proceedings of the Institution of Civil Engineers, December, 1903. 

 Dr. Stanton used this gage in his experiments on the air resistance of 

 small plates. 



The principle of the gage is that of the inclined liquid U-tube, but 

 instead of giving the tube an initial pitch and observing the change of 

 level of the liquid, the Chattock gage is titted with an elevating screw 

 and micrometer by which the gage is tilted to balance the pressure 

 difference in its two ends. By reading on the micrometer the amount 

 of tilt given, the head in inches of liquid is computed. By this means 

 there is no motion of the liquid in the glass, and errors due to capil- 

 larity and viscosity are eliminated. Furthermore, the condition of 

 the surface of the glass has no effect. 



The gage (pi. 5 and fig. 2) consists of a glass U-tube moinited on a 

 tilting frame T. The pressures to be measured are connected to the 

 bulbs A and C, which are in communication with each other through 

 a horizontal tube bearing a third bulb B at any intermediate point. 

 The bulbs A, C and the lower part of B are filled with water. The 

 upper part of B is filled with castor oil. The water in B and C is in 

 free communication and hence the oil in B is at the pressure of C. 

 The water in A is led through a thin walled tube through the bottom of 

 B extending into the castor oil. An excess of pressure in A over the 

 pressure in C will cause water to flow from A into B. A water bubble 

 will then grow at D and expand into the oil. The gage can be tilted 

 so that this bubble remains of uniform diameter. The pressures in 

 A and C are then balanced. To provide this tilting the manometer 

 proper is mounted on a tilting frame T, which pivots on the knife 

 edges at G and is elevated by the screw F . The whole is carried on a 

 bed frame Z fitted wMth three leveling screws /, a retaining spring 

 H, and a scale S, on which may be read the full tt-.rns of the screw F. 



A microscope, M, fitted with cross-hairs is mounted on the frame T 

 and directed at the bubble B. A small mirror on the opposite side 

 illuminates the surface of the bubble. 



The screw F is fitted with a large drum divided into 100 parts. 

 The screw has 20 threads to the inch. The gage is sensitive to one- 

 half of a division on the drum, and hence to a movement of the screw 

 of 1/4000 inch. 



