﻿o7C Prof. L. Vessot King on Precision Measurement of 



The non-compensated Kelvin Bridge and anemometer can 

 •easily be employed to analyse gas-temperatures as well as 

 velocities. The special feature of these connexions already 

 mentioned eliminates the error due to variation of tempera- 

 ture along the length of the current- and potential-connecting 

 vires and the same advantage is obtained as in Oallendar's 

 system of compensated leads iu platinum thermometry. 

 When using the anemometer-wire as a linear thermometer, 

 it is necessary to employ a very small measuring-current 

 and a more sensitive galvanometer than in velocity measure- 

 ments ; the resistance B is reduced to the proper value by 

 shunting by means of a suitable manganin resistance, and 

 fine adjustments are made by including in parallel an adjust- 

 able high resistance such as that which forms the essential 

 part of the Kelvin Varley potentiometer. If temperature 

 distribution is to he measured in a stream of air at high 

 velocities correction should be made for the "'kinetic 

 heating," i. e., the heating-effect due to the impact of the 

 air-molecules on the wire. From some experiments of the 

 writer, this kinetic heating-effect is roughly proportional to 

 the square of the velocity, and is about 1° Cat 1500 cm./sec. 

 in the case of a 2-J mil-wire*. If very accurate tempera- 

 tures are required the measuring-current should be varied 

 and temperatures extrapolated to zero current. By pro- 

 ceeding in this manner the writer has succeeded in measur- 

 ing temperature as well as velocity distributions to 1/100 of a 

 degree C., making use of a galvanometer of sensitivity 10~ 9 

 ampere. This additional property renders the hot-wire 

 anemometer useful in investigating conditions of heat-transfer 

 from gases and liquids to solid surfaces ; data on these 

 points "rtould be of importance in many technical problems 

 relating to methods of air-cooling ot internal-combustion 

 engines, oil-cooling of electrical transformers, and the ven- 

 tilation of electrical machinery. 



In connexion with the modern types of oscillograph the 

 employment of the anemometer in the measurement of 

 variable gas velocities and temperatures might furnish results 

 of value in many engineering problems. 



In technical problems relating to the measurement and 

 recording of gas-flow in pipes, the anemometer employed in 

 the form considered would seem capable of useful develop- 

 ments ; its property of measuring the mass-flow at all 

 pressures has already been mentioned. In particular, the 

 application to the measurement of steam-flow would give 



* Compare the experiments of Joule and Thomson, Trans. Roy. Sec. 

 June, 1860; Kelvin, 'Collected Works,' vol. i. pp. 900-914. 



