﻿the Magnesium High- frequency Arc. 



97 



spectrum is gradual, not in the sense that the new spectrum 

 continuously strengthens at the expense o£ the old one, but 

 as the frequency is increased the spectrum tends to become 

 unsteady, the continuous spectrum disappears and new lines 

 come up intermittently ; as the frequency is further increased 

 the second spectrum predominates, and tbe first one comes 

 up intermittently, finally disappearing as the frequency is 

 further raised. 



If an Oudin coil in the form of a solenoid of 2500 turns 

 of 3*4 cm. diameter and 1 metre long is placed between the 

 oscillatory circuit and the arc, thus raising the voltage and 

 diminishing the current strength, the character of the 

 spectrum is again changed. The nitrogen band spectrum is 

 visible and the metal lines weakened even at the low frequency. 

 With a high frequency and high voltage tbe spectrum consists 

 of the band spectrum, the u b" triplet^ and \4481. 



That the current density is an important factor in the 

 changes is suggested by the effect that the shape of the elec- 

 trodes has upon the spectrum. At the low frequencies 

 pointing the electrodes causes only an increase in the con- 

 tinuous spectrum, but at the higher frequencies it tends to 

 accentuate the characteristic appearance of these spectra. 

 Owing to the wearing away of the electrodes it was necessary 

 to file the ends periodically, but care w T as taken to keep the 

 shape as nearly constant as possible during the experiments. 



Photographs were taken of the spectra obtained w r ith 

 various values of inductance and capacity in the oscillatory 

 circuit, with and without the Oudin coil in the arc circuit. 



In examining the photographs the instability of the spectra 

 in certain cases must be remembered, the photographic record 

 being often of an integral nature, and it is possible that lines 

 which come Up intermittently but brightly may make a 

 greater impression than feebler but more persistent ones. 



The following table shows the values of the inductance, 

 capacity, and frequency in the oscillatory circuit for the 

 various photographs :— 





Inductance in 

 centitnetves. 



Capacity in 

 microfarads. 



Vibrations 

 per second. 



d, ? ... 



168,200 



: 



82,410 

 30,710 

 58,490 



0'01245 

 00020 

 0C0101 

 0020 



000101 



1-117 XlO 3 



2'787 XI 5 

 8-921 x 10 5 



(V424X10 5 

 6*66 X 10 s 





q t I .... 



h, m 



i, n .< 



Phil. Mac/. S. (5. Vol. 23. No. L33. Jan. L9-12, 



1 



