140 PHENOMENA, ATOMS, AND MOLECULES 



1330 watts per sq. cm. delivered by the atomic hydrogen flame, the tem- 

 perature of a black body would rise to 3900° K. The power radiated from 

 tungsten at its melting point is 395 watts per sq. cm., and 1330 watts per 

 sq. cm. should heat tungsten to about 5300° K. Although tungsten wires 

 and even sheet metal can be melted and welded by the atomic hydrogen 

 flame, it does not occur so rapidly as the foregoing figures would lead us 

 to expect. 



At such high temperatures, however, the rate of surface heating by an 

 atomic hydrogen flame must decrease because of the fact that the hydrogen 

 remains partly dissociated so that the recombination is not complete. With 

 surface temperatures below 2000° K., however, this factor would be 

 negligible. It is probable that the rate of surface heating would be depend- 

 ent not so much on the temperature gradient in the surface film of gas 

 as on the rate of diffusion of atomic hydrogen through this film. Thus we 

 may expect the rate of delivery of energy to a metal surface to remain 

 nearly constant until the surface reaches a temperature of at least 2000° K. 



To test this conclusion and also to find out whether different metals are 

 heated at different rates, another device like that shown in Figure 2 was 

 constructed, but the tip which was heated was of copper instead of tung- 

 sten, being made by turning down a large block of copper. When energy 

 is delivered at the rate of 1000 watts per sq. cm. the temperature gradient 

 in the tungsten tip is about 1100° K. per cm., while with a copper tip it is 

 260° K. per cm. Thus the surface of the tungsten tip was probably at a 

 temperature of at least 1400° K. while the copper surface was not over 

 600 °K. The observed rates of surface heating with the copper and 

 tungsten tips were, however, not measurably different. This seems to 

 indicate that up to 1400° K. the temperature of the surface does not affect 

 rhe rate of heating and also that copper and tungsten are equally effective 

 ui causing recombination of hydrogen atoms on their surfaces. 



TOTAL HEAT DELIVERED TO SURFACES 



The foregoing method measures the intensity of surface heating in 

 watts per square centimeter. It was of interest to determine what fraction 

 of the total energy in an arc or a flame could be delivered to a large flat 

 surface against which the flame was directed. For this purpose a cylinder 

 of copper 10.5 cm. in diameter and 9.8 cm. long was used, which weighed 

 7950 grams. The flame was directed against one of the flat polished ends, 

 and the rate of temperature rise was measured as before. 



An atomic hydrogen flame was produced by a 60-ampere a. c. arc 

 using a torch like that shown in Figure 3. The voltage across the electrodes 

 was 70 volts. A wattmeter showed that the power consumption in the arc 

 was 3510 watts, which gives a power factor of 0.84. The electrodes were 



