SELECTION OF MATERIAL, APPARATUS, ETC. 7 



by more than o.oi ampere. The current was usually 0.8 ampere or a little 

 less. The ammeter was appreciabl}' more sensitive to small changes in the 

 terminal voltage than a comparably graduated Thomson A. C. voltmeter, 

 because the current shunted through the voltmeter was not negligible in 

 comparison with the current which fed the glower. Among other sources the 

 electric arc was given a fair trial and discarded for two reasons, first, because 

 of the intensity of the carbon and C3'anogen bands, and second, because of 

 the inconveniences resulting from its unsteadiness and great emission of heat. 



For wave-lengths between the strong ultra-violet of the Nernst glower 

 and o. 2,a a spark discharge in air of about i cm. length was used. In obtain- 

 ing the greater number of the negatives one electrode was composed of an 

 alloy of equal parts by weight of cadmium and zinc and the other was made 

 of sheet brass. The alloy wore away so rapidly that the brass electrode was 

 employed to reduce the labor attendant upon sharpening the terminals. 

 The electrodes were given a form apparently not 

 described before. As is very well known, manj^ 

 spectral lines, both weak and strong, produced 



b}^ sparks between metallic surfaces extend only :^"'"~~"tii^' ^ 



a short distance beyond the metal and hence do '''^i^::\f^h::^:'^ii;i ^p'^''^ 



not offer a continuous source of light! across the t " ' r'' (V 



entire spark gap. In order to obtain a back- 

 ground of uniform intensity from edge to edge 

 of the negatives it was necessary' to use some Fig. 1. Flat and edge views, 



scheme to nullify the effects of the non-uniformity of emission in the spark. 

 One way of accomplishing this is to rapidly translate the electrodes (main- 

 tained at a fixed distance apart) back and forth parallel to the length of 

 the slit of the spectrograph by some mechanical device. 



The reciprocating action associated with this plan shakes the camera 

 and grating to such an extent as to demand greater rigidity in the apparatus 

 than it usually has. Therefore the electrodes were made in the shape of 

 wedges or chisels with the sharp edges parallel to the slit. The well-known 

 distribution of a rapidly alternating current in a conductor necessitated 

 curving the edges of the electrodes, as is shown in fig. i, which is Ji natural 

 size. Due to the tearing away of the metal, and to various other causes, 

 the innumerable thread-like sparks changed the positions of their ends so 

 rapidly that the integrating action of the photographic film recorded a 

 perfectly uniform negative for exposures of 15 seconds or more. The 

 exposures generally lasted 75 seconds. The electrodes had to be kept sharp 

 and smooth, for, when this was neglected, the elementary sparks persisted 

 much longer in one position than in another and consequently caused streaks 

 of varying intensity to run along the negatives parallel to their length, as 

 can be seen in some of the positives reproduced in the appended plates, e.g., 

 fig. 99, plate 25. 



