74 



KENNELLY-SANBORN— INFLUENCE OF 



[April 24, 



In the direct-reading method, the connections are as shown in 

 Fig. 13. Here the test-wire is connected across iio-volt Hghting 

 mains, through an adjusted resistance, which may consist of incan- 

 descent lamps, so as to receive as nearly a constant heating current 

 as is practicable. The voltmeter V is connected to the potential taps, 

 15 cm. apart on the vertical test-wire. The apparatus is then set up 

 at the place where the wind is to be measured. The four leads are 



i: 



kv^ 



ii. 



Fig. 13. Connections for Direct-Reading Type of Anemometer. 



of any convenient length, and are bound up into a weatherproof cable. 

 From calibration measurements made on a sample of the test-wire in 

 a motor-driven fork, the linear convective dissipation of heat for any 

 safe given linear resistance of the wire is known. As the horizontal 

 component of wind-velocity increases, the temperature of the plati- 

 num test-wire falls, since no provision is made in this case to restore 

 the initial temperature. A calibration curve has therefore to be pre- 

 pared for a given exciting current, whereby the readings of the volt- 

 meter, which may be a recording instrument, become convertible into 

 wind-velocities. A set of calibration curves is given in Fig. 14 to the 

 left-hand scale, both for 1.5 amperes and 2.0 amperes, constantly sup- 



