1914-1 



ATMOSPHERIC PRESSURE. 



/o 



plied to the test-wire from the i lo-volt circuit. It will be seen that 

 when the wire is greatly cooled by the wind, a very appreciable cor- 

 rection for the temperature of the wind enters into the result; 

 although at a temperature elevation of say 300^ C, this correction 

 would be comparatively small. It is necessary to set the current at 

 such a value that when the wind fails, the test-wire shall not be 

 dangerously heated. It was found that with a platinum test-wire of 

 0.1 14 mm. diameter, as used in these measurements, 1.5 amperes was 

 a suitable current for wind-velocities up to 15 km. per hour. Thus, 

 as indicated in Fig. 14, with a wind of 10 km. per hour, and a temper- 

 ature of 10'' C, the voltmeter reading was 4.3 volts. When, how- 

 ever, the wind-velocities were higher, the current was increased to 2 

 amperes, which, in still air, raised the wire temperature to visible red- 

 ness. At 30 km. per hour, and 10^ C. wind temperature, the p.d. on 

 15 cm. was then 5.8 volts. 



■"^ i'"^-^ 5 ^'•" ? s » ? I 5 e I I 



Fig. 14. Calibration Curves of 15° cm. Test Wire. 



The G-support and test-wire were fastened to a pole and sup- 

 ported out of doors, exposed to the weather for some weeks. The 

 apparatus appeared to be durable, and constant in its indications 

 throughout that time, except that the solderings of the platinum tap- 



