the Thermometric Anemometer. 691 



intervals of 120° C. in the ebonite ring B. These served for 

 the insertion of the heating element in the flow system. This 

 was effeeted by means of brass rods passing through the holes 

 H and through similar holes drilled in ebonite rings attached 

 to the main flow tube. The whole was securely fixed by means 

 of nuts on the ends of the brass rods *. The heating element 

 was inserted in a vertical plane in the horizontal flow system 

 with the several wires aboutequally inclined to the vertical as 

 shown in fig. 1, so that the free convection currents tended 

 to equalize the temperature distribution across the section of 

 the How tube. The construction of the two platinum thermo- 

 meters employed resembled that of the heating element : 

 there was in this case, however, no necessity to employ an 

 inner brass ring, and the pins for the support of the platinum 

 wires were made considerably smaller than in the case of 

 the heating element. The thermometers were inserted in 

 the flow system in a similar manner to that employed for the 

 insertion of the heater. The distances b-tween the respective 

 thermometers and the latter could be adjusted to any desired 

 values by the use of suitable lengths of tube provided with 

 ebonite ends affixed to the tube and suitably bored. For 

 very small distances between heater and thermometer, thin 

 separating disks of ebonite bored to the appropriate diameter 

 were similarly employed. The thermometers were calibrated 

 bv the determination of their respective resistances at 0°C. 

 and at a temperature of 50° + '02 w C, on gas scale, corre- 

 sponding to 49-62 + 0-02° C. on Pt scale (5 = 1-5), this 

 temperature being maintained thermostatically in a water 

 bath, and determined by a mercury thermometer standardized 

 at the N. P. Laboratory. The thermometers were adjusted 

 as nearly as possible to equality of resistance at 0° C, 

 and they were, in use, connected differentially with a 

 Callendar and Griffiths' bridge, the scale of which was 

 provided with a vernier, enabling the balance to be deter- 

 mined to 0*01 cm. The supply of energy to the heating 

 element was determined from a knowledge of the current 

 supplied and the drop of potential occurring across the 

 heater. The current was read from a Siemens & Halske 

 ammeter reading to 0001 amp. The voltage drop across 

 the heater was determined by means of a Ravleigh pote< tio- 

 meter composed of two P 0. Boxes adjusted so that their 

 total resistance was 10,000 ohms throughout. The potentio- 

 me'er readings were calibrated by means of a Weston cell 

 standardized at the N. P. Laboratory. 



* See Phil. Mag. vol. xliii. p. 279 (192iT) . 

 2 Y2 



