Effect oj rressure iipon Arc Spectra. 573 



Copper, 



No very marked differences from the normal arc spectrum 

 are noticeable in the intensity and character of the copper 

 arc as photographed at 40 atmospheres pressure in air ; the 

 spectrum corresponding line for line with that given by 

 Kayser and Runge ; it is, therefore, not necessary to repro- 

 duce the spectrum. 



An alternating current of 40 amperes and 320 volts was 

 used. 



Iron Arc-Spectrum, 



Photographs of the iron arc were taken at 0*08 and 44 

 atmospheres, in each case with a comparison spectrum of the 

 iron spark with self-induction and capacity. The relative 

 intensity and general character are closely similar to the arc 

 at atmospheric pressure as shown in the map of Kayser 

 and Runge. 



A point noticeable about the 44 atmosphere arc is that a 

 number of the lines in the ultra-violet above 3800 are very 

 strongly reversed. 



Since these experiments were carried out an excellent 

 paper on this subject has been published in the Astrophysical 

 Journal *. Prof. Hale has used for this work a much higher 

 dispersion than was employed in our experiments, and 

 promises a further communication on the subject. It is 

 therefore not necessary for us to deal further with this 

 effect. 



Iron Gloiv- Spectrum. 



The spectra which we are about to deal with were taken in 

 the large steel enclosure described above, the electrodes 

 consisting, the one of a 1^-inch iron bar, the other of a |-inch 

 iron rod. The whole apparatus was exhausted. It was then 

 noticed that when the length of the arc was increased to 

 anything above 2 or 3 mm., the nature of the discharge 

 completely changed. With the 2 mm. arc a thin light blue 

 line was seen to pass in a practically straight line between 

 the points of the two electrodes; the spectrum given under 

 the conditions is substantially similar to the iron arc at all 

 other pressures, and will not therefore be further discussed. 



So soon as the distance between the electrodes was 

 increased the whole gaseous mass contained in the enclosure 

 commenced to glow brightly, the discharge no longer leaving 

 the electrodes at their nearest points but being uniformly 

 distributed over the last 3 or 4 inches of their length. 



* Hale aud Kent, Astropliys. Journ. xvii, p. 154 (1903). 



