APPARATUS AND METHODS. 1 1 



off rapidly by the strong transformer discharge and it was necessary to clean these off every few minutes 

 and also to tile off the oxide from the iron tip. Titanium terminals wore away rapidly, owing to disin- 

 tegration of the metal, and the oxide also needed to be removed frequently if the brightest discharge was 

 to be obtained. The short spark gap necessitated an auxiliary gap in series, as otherwise the discharge 

 was not sufficiently disruptive to avoid melting the terminals. This auxiliary gap was a simple affair 

 of brass mounted on fiber. 



When using the spark, the various parts of the secondary circuit, as well as the step-up connection 

 and the current in the primary, were adjusted to give the sort of spark desired. In this investigation 

 self-induction has been used in the spark circuit somewhat sparingly, since on the majority of the photo- 

 graphs it was necessary to obtain the fainter lines of sufficient strength for accurate measurement. Self- 

 induction in the spark circuit sharpens the Zeeman components in about the same degree that it sharpens 

 the lines of the regular spark spectrum, but the brightness of the spark is greatly diminished at the same 

 time, an eflect onl)- partially due to the decrease in intensity of the enhanced Unes. The weaker lines as 

 a whole, especially the faint and diffuse lines of iron, are so reduced by self-induction that very long 

 exposures are required to bring them out. A compromise must be made, since in exposures running 

 many hours, especially for more than one day, there is risk of instrumental disturbances. The method 

 followed was to use the spark with rather high self-induction for one or more photographs of any region 

 containing strong lines, and especially enhanced Knes, for which moderate exposure time was sufficient, 

 then to use small self-induction for photographs in which as many of the weak lines as possible were 

 desired. The loss of sharpness in such cases was counteracted as far as possible by the use of a narrow 

 slit and by selecting the kind of plate and developer which would give the sharpest definition and at the 



same time show the Hues. 



2. The Electro-Magnet. 



This apparatus is of the Du Bois half-ring type, made by Hartmann and Braun of Frankfort. It is 

 shown (in its present state, after being rewound) in the photograph of the laboratory (Plate I). The 

 coils, as used until recently, were each wound with 1250 turns of No. 9 wire (diameter =3.0 mm). They 

 are clamped to a horizontal iron base which completes the magnetic circuit. The magnetic gap is varied 

 by mo\nng the coils upon this base, which is itself supported by three legs on an iron plate. A hole in 

 the center of this plate fits over a pivot in the middle of a round iron table, the ends of the plate resting 

 on a planed ring which forms the rim of the table. The magnet can thus be turned in any desired direc- 

 tion by rotating the base-plate upon the planed ring of the table. The magnet rests upon a cement pier 

 60 cm square and 82 cm high. The core of each magnet coil is pierced by a horizontal hole 17.5 mm in 

 diameter for the transmission of light along the lines of magnetic force. These holes are filled with cylin- 

 drical iron rods when such an axial opening is not needed. 



A variety of pole-pieces was used for the magnet according to the way in which the spark terminals 

 were arranged and the directions in which the light was to be taken. Into each vertical face of the magnet 

 core is screwed the first section of the pole-piece, a truncated cone of soft iron 16.5 mm thick, whose 

 double angle is 112. The small end of this cone is a circular plane surface 39 mm in diameter. To this 

 circular face was fastened a pole tip of one of the following forms, each of which has a double angle equal 

 to that of the truncated cone just described. 



(a) For the observation of the fight from the iron spark parallel to the lines of force, the magnet 

 poles themselves were used as spark terminals in some of the earlier experiments. In this arrangement 

 the faces of the tips were circular, of 6 mm diameter. One pole was left solid and the other pierced with 

 a hole 3 mm in diameter, the spark being viewed through the tubular hole in the core. The pole-tips 

 were each insulated from the core by mica plates and held in place by fiber screws. The method gave 

 trouble, not only from the occasional breaking down of the insulation, but from the fact that the spark 

 did not stay in front of the hole in the pole-piece. It had the advantage, however, that the field was not 

 affected by the introduction of extra iron as spark terminals. 



