Arthur Edward Ruark

Helmholtz Coils for Producing Uniform Magnetic Fields

The “Helmholtz coil” is really a pair of coils so proportioned as to produce a magnetic field uniform in magnitude and direction over a large region. The theory of the magnetic field produced by such a coil is briefly developed and the final (series) formula is put into a form convenient for calculations. For ordinary applications only the field at the center is required. A convenient design, developed for obtaining soft x-ray potentials by determining the velocities of ejected photo-electrons, is described, and is further explained by photographs and drawings.

CII. Fine structures of spectrum lines

Abstract Many spectral lines of non-hydrogenic atoms have fine structures which arise from transitions between the components of complex spectral levels. In none of the cases so far studied can fine structures be due to isotopy. Elements which are known to have only a single species of nuclens frequently have complex spectral lines. In general, some of the satellites which can be predicted from the energy diagram do not appear. In the case of thallium a fine quantum number f may be introduced, with a selection principle to govern its changes, but for other elements, no selection principle has been discovered. Fine structures have proved to be an aid in classifying spectral lines. At present it is believed that slight quantized variations in the configuration of loosely bound underlying electron shells suffice to explain the existence of all the fine structures we have studied.

Multiple Electron Transitions and Primed Spectral Terms

Excitation of the lines of Mg+in low-voltage, low-current discharge. The stronger lines of Mg+ appear at potentials equal to the ionization potential of Mg plus the excitation voltages predicted from their classifications. This proves that a single impacting electron can ionize an atom and raise a valence electron of the ion to a higher orbit.Excitation of the pp′ lines of Mg and Cd in low-voltage, low-current discharge. Using a quartz spectrograph to analyze light from the force-free space between grid and plate, it was found that the Mg pp′ group (2776 to 2783 A) appears at current densities less than 0.2 milliampere per square centimeter. Since these lines are produced by electron transitions from an initial state in which there are two electrons in virtual orbits, this observation shows that the two electrons are moved to these orbits as a result of a single collision; lines which could be produced only by successive impact are entirely absent at much larger current densities. The pp′ group of Cd (2239 to 2329 A) appears at its quantum voltage, but is too faint to be detected at current densities which exclude the possibility of successive excitation.Several lines of Cd, In, and Tl are classified by the use of primed terms.Extension of the subordinate series of Cd: spectra of Cd below the ionization potential. The low-voltage arc in Cd yielded a number of new lines of the singlet and triplet subordinate series. In exposures taken below the ionization potential the lines of Cd are excited exactly at the voltage intervals predictable from the classification of the spectrum.

Stages in the Excitation of the Spectra of Thallium

Critical potentials of thallium vapor have been measured with the following results—first resonance potential 0.9 volts, first ionization potential 6.04 volts. A higher critical potential detected by the photoelectric effect of the radiation is at 12.4 volts.Stages in the development of the spectrum were as follows. Below the ionization potential successive lines of the two subordinate series appear with increasing voltage. Above 12 volts another line spectrum appears.Comparison of the critical potentials with the doublet series system of thallium shows that the first ionization potential is determined by the limit 2p2 and the first resonance potential by the difference 2p2–2p1. Since no radiation can result from this transition the orbit 2p1 must be meta-stable. Resonance potentials corresponding to frequencies 2p2–ms and 2p2–md give rise to the observed low voltage spectra. Corresponding critical potentials are computed for Al, Ga and In.The 12.4 volt critical potential and the associated second line spectrum are ascribed to a second type of single ionization, the removal of one of the two 61 electrons.

The Spectra of Fifth Group Metals

WE have photographed the absorption spectrum of bismuth and also the spectrum of the thermionic discharge at potentials ranging between 4 and 60 volts. Several stages in the excitation of the arc spectrum, and at least two classes of spark lines, have been recognised; 64 arc lines have been classified. The spectrum of the neutral atom is characterised by wide doublets, and most of the energy-levels so far identified are of p-type.