the Hot-wire Anemometer. 509 



other gas is passed through the gas-meter M, and thence to 

 a wide tower loosely packed with lumps of calcium chloride. 

 A thermometer T is placed in the stream at a point just prior 

 to the gas entering the brass pipe P in which the anemometer 

 A is inserted, a gas-tight junction between the ends of the 

 anemometer tube and the remainder being effected by means 

 of carefully made spigot unions, constructed so that no dis- 

 turbance of the stream is introduced thereby. No appre- 

 ciable error could be attributed to irregular rotation of the 

 drum of the meter. Even at low velocities extremely steady 

 velocities were indicated by the hot-wire anemometer. It 

 is essential to employ water free from any dissolved coal- 

 gas or similar gas in the meter as the indications of the 

 hot-wire anemometer in the form described are extremely 

 erratic in the presence of small quantities of coal-gas &c. 

 The meter was therefore thoroughly washed out with water. 

 The stream of air was derived from a gas-holder having a 

 volume of 5 cubic feet. The pulley over which passed the 

 supporting rope of the bell, the other end of which was 

 weighted, was provided with a cam over which passed a rope 

 carrying a compensating weight, so that the same pressure 

 was maintained throughout the fall of the bell in its tank. 

 Any desired pressure could be obtained by suitably weighting 

 the bell. It was found that the desired constancy of pressure 

 could be readily obtained by the use of this pressure device, 

 without the introduction of any further pressure governors 

 in the circuit. The precautions already detailed were taken 

 to ensure the freedom of the water in the holder from any 

 dissolved coal-gas. In calibration of the anemometer, air 

 was passed through the apparatus, and the rate determined 

 by observation of the pointer of the gas-meter, this obser- 

 vation extending over a period varying with the velocity 

 of flow in the pipe, so that with low velocities the period of 

 observation was proportionately longer. A length of about 

 five feet of tube was situated on either side of the anemo- 

 meter wires, and the internal surface was throughout the 

 whole length of the same made as smooth as possible by 

 polishing. Usually the whole length of tube was wrapped 

 with asbestos cord. A number of fine copper gauzes was 

 introduced into the flow-tube at G, so as to effect distribution 

 of the current of gas across the section of the tube. The 

 meter M was of the type customarily employed in the 

 technical practice of gas measurement. It was made by 

 Sugg, and one rotation of the drum corresponded to the 

 passage of 1/12 cub ft. of gas. This volume was adjusted 

 accurately to the desired amount in the usual manner by the 



