242 Dr. J. S. G. Thomas on Thermal Effect of a slow 



into the* ebonite in the manner shown at la. Alternate 

 bushes were inserted from opposite sides o£ each ebonite 

 block. The ends of the copper wires were connected to 

 binding screws affixed to ebonite blocks B secured to the 

 anemometer tube as shown, so that each of the platinum 

 wires inserted in the tube was connected with very approxi- 

 mately the same length of lead. The several terminals on 

 the ebonite blocks were connected separately by means of 

 thick stranded copper wire with terminals dipping into 

 mercury cups, so that any or all of the heated platinum 

 wires could be inserted in any external circuit as desired. 

 The insertion of the tube in the main flow tube was effected 

 by means of the spigot unions shown at IS, S, a device 

 affording a smooth junction between the tubes, with the 

 prevention of eddies. The anemometer tube was wrapped 

 in three layers of felt, affording efficient thermal insulation, 

 and the remainder of the flow-tube wound with asbestos 

 cord. The general method of calibration of the heated 

 wires for purposes of anemometry is given in the papers by 

 the author already referred to*. The fixed ratio arm in the 

 Wheatstone bridge employed was throughout adjusted to 

 1000 ohms. For purposes of measuring the respective 

 temperatures to which the several wires were raised by 

 the heating current and the air stream employed, the 

 respective values of E for the individual wires were de- 

 termined from their respective resistances, measured at 

 atmospheric temperature, using the value of the temperature 

 coefficient of the sample of wire from which the several 

 wires were cut, determined by means of a Callendar and 

 Griffiths bridge. The current used for this purpose was 

 0*005 amp., and allowance was made for the lead resistance. 

 When the platinum wires were heated by the current (1*1 

 amp.) employed in the series of experiments detailed below, 

 their respective temperatures were deduced from their re- 

 sistances as calculated from the voltage drop occurring 

 across the respective wires. The platinum-scale temperatures 

 so calculated were in all cases corrected to the gas scale. 

 The Weston voltmeter employed enabled readings to be 

 made to 0*001 volt, and it was calibrated before use. Head- 

 ings of current were made either on a calibrated Weston or 

 Siemens & Halske milliammeter. In the present series of 

 experiments the anemometer wires were used in a hori- 

 zontal position, and the flow-tube was likewise placed 

 horizontally. 



* Phil. Mag. loc. cit. Proc. Phys. Soc. loc. cit. 



