120 PHENOMENA, ATOMS, AND MOLECULES 



sociation is nearly complete and there is thus no concentration gradient. 

 Let us assume that heat is carried in this region by conductivity through 

 atomic hydrogen gas. From the principles of the kinetic theory we can 

 calculate for a monatomic gas of atomic weight M at very high tem- 

 peratures that 



(^ = 5.6X10--° 7V-i/(o-2^/M) 



where o is the diameter of the atoms of the gas. Taking o = 1.3 X io~^ cm. 

 as a reasonable estimate of the diameter of the hydrogen atom, we find for 

 atomic hydrogen 



c/> = 3.3X10-'' 73^2 



Placing To = 10,000 and Ti = 5,000 we find </>2 — <^i = 215 watts per cm. 

 and thus from the value of Wd we find by equations (10) and (14) that 

 a/r is 3.0. Thus between a cylinder of 0.5 mm. and one of 1.5 mm. diameter 

 1500 watts per cm. can be conducted by the atomic hydrogen with a drop 

 in temperature from 10,000 deg. down to 5000 deg. From this cylinder of 

 1.5 mm. the diffusion of atomic hydrogen could carry the energy out to 

 one of 6.0 mm. diameter, while the temperature falls to a point where there 

 is little dissociation (say 2000 deg.). 



Thus the diffusion of atoms from the arc is an adequate mechanism for 

 carrying the energy out to a distance of 3 mm. from the arc. We still have 

 to explain how the heat can be transferred from this point to the bulb. 

 We have seen that neither convection nor conduction through the molecular 

 hydrogen can carry this amount of energy. 



We are therefore forced to the conclusion that in this arc the heat was 

 delivered by the atomic hydrogen directly to the heavy tungsten electrodes. 

 The length of the arc was only 7 mm. and one of the electrodes was a solid 

 piece of tungsten about 15 mm. in diameter. ^^ 



We must also conclude that the high energy of 1500 watts per cm. was 

 only possible in this arc because of the proximity of the mass of tungsten. 



PRELIMINARY EXPERIMENTS WITH FLAMES OF 

 ATOMIC HYDROGEN 



To try out the possibility of blowing atomic hydrogen out of an arc, 



20-amp, arcs from a constant-current transformer were passed between 



two tungsten rods 6 mm. in diameter mounted transversely in a horizontal 



alundum tube (10 cm. diameter) through which a stream of hydrogen 



flowed. With voltages from 300 to 800, arcs could be maintained with 



electrode separations up to 2 cm. The magnetic field of the arc caused it 



*^ A photograph of this arc, showing the electrodes, is given in Mackay and 

 Ferguson's article. See reference 14. 



