510 Mr. J. S. G. Thomas on 



use of the 1/12 cub. ft. bottle specified in the Notification 

 of the Metropolitan Gas Referees. The volume could be 

 adjusted with certainty to 1 part in 400, and with very little 

 change from day to day. The movement of the indicating 

 hand of the meter was very regular, even when revolving 

 at extremely low velocities. 



A definite current — in different experiments adjusted to 

 values between 0*6 amp. and 1*5 amp. — was maintained in 

 the bridge, the constant-ratio arm being adjusted to 1000 or 

 2000 ohms. The bridge was first balanced with zero flow of 

 air, the current being adjusted to the specified value. The 

 air current was now passed through the tube, and the deflexions 

 of the galvanometer observed on both sides of the zero after 

 adjustment of the current to its initial value. The galvano- 

 meter employed was either a unipivot pointer instrument by 

 Paul of resistance 60 ohms, or a suspended-coil reflecting 

 galvanometer whose sensitiveness was suitably adjusted by 

 the use of shunts, or by the insertion of resistances in series 

 with the coil. The general nature of the results was the 

 same in all cases. The deflexion having been read, the drop 

 of potential across the shielded wire was determined as 

 already explained, and the bridge restored to balance by 

 adjusting the resistance in the fourth arm of the bridge, 

 the electric current being maintained constant throughout. 

 The temperature of the air-stream was determined and 

 likewise the temperature of the air in the gas-meter, and 

 the barometric pressure. The excess pressure at the meter 

 outlet was observed. In no case did this exceed 0*8 inch 

 of water. The method of reducing the readings is deter- 

 mined by the fact that, as pointed out by King*, his own 

 theoretical investigations, and the experiments of Kennel ley 

 and Samborn f show^ that the hot - wire anemometer 

 measures the mass-flow of the gas. The readings of gas 

 volumes were therefore throughout reduced to 0° C. and 

 760 mm. pressure. The necessary correction of velocity 

 for pressure has been shown by King to be given by 



^ r = — -- ±, and for ordinary variations is negligible. 



Amongst other disturbing factors which might affect the 

 anemometer readings, mention may be made of the effect 

 of variation of temperature and the transverse vibration of 

 the wire upon the observed reading. Variations of tempe- 

 rature of the stream of fluid are not entirely eliminated by 



* Phil. Mag., xxix. p. 570 (1915). 



f Proc. American Phil. Soc, viii., (1914). 



X King, Phil. Mag., he. cit. 



