Das Verhalten der thermoelektrischen Effekte in tiefsten Temperaturen

The thermoelectric phenomena of electronic conductors are considered using only the 3 principal laws of thermodynamics. As a consequence of the Nernst-theorem the product of electrical conductivity [sigma] and differential thermoelectric force must vanish for T -> 0. Only if lim [sigma] T->0 =/= 0, one must conclude, that the thermoelectric force disappeares in the limit T -> 0. If lim [sigma] = 0 (for instance for some semi-conductors) the theorem of Nernst gives no essential rule for the behaviour of the differential thermoelectric force at T = 0. If the Thomson coefficient [my] is given by [my] = [beta] x Tn in the neighbourhood of T = 0, the Nernst-theorem gives for the case lim [sigma] T->0 =/= 0 the condition n > 0. The hitherto accepted condition was n > 1. Thus the present theory explains the hitherto reported discrepancies between the thermodynamical and the electron theoretical treatment of the thermoelectric phenomena

Einfluß der Austauschwechselwirkung auf die Wärmeleitfähigkeit von Metallen

The electron theory of metals predicts, that for T>[Theta]D the Wiedemann-Franz number L has the universal value 2,445x10^-8 (Volt/Grad)^2. For the alcali-metals Na and K the experiment value for L is nearly 10% lower. As explanation for this fact the influence of electronic exchange-forces is considered. These forces diminish the electronic heat below the Sommerfeld value. For description of the potential between two electrons the form e^2/r exp (-[my]r) is used. In the case of Na good numerical agreement with experiments is obtained if the value of [my] is taken from Landsberg's theory of X-ray emission-bands

Adolf Scheibe

Nachruf der Braunschweigischen Wissenschaftlichen Gesellschaft vorgetragen in der Sitzung am 16. Juli 195

Zur Schallausbreitung in verdünnten Gasen

Dispersion of sound in rarified monoatomic gases is calculated from a kinetical point of view, using the approximation of Burnett-Chapman. The resulting dispersion-effect is greater by about 50-70% than the hydrodynamical dispersion by viscosity and conduction of heat. This effect is, in agreement with similar considerations of Wang Chang & Uhlenbeck, only little sensible against changes of the model of molecule

Der zeitliche Temperaturverlauf in elektrischen Kontakten

R. Holm has given a simple calculation of the temperature in electrical switching cantacts assuming a spherical symmetry of the effective contact area. Here the present authors give a more precise derivation of the temperature dependence on time assuming a contact model better corresponding to the technical conditions. Thus the effective contact area is assumed to have cylindrical symmetry. In consequence the calculations become more difficult. The equations are solved in terms of the Green functions of the thermal conduction formulae. The authors solve the integrals by numerical methods. These results are compared with the above mentioned simple formulae of Holm and it is shown that there exist strong deviations especially immediately after switching in the current; this temperature rise amounts to about 40% of the previous formulae only

Weglängenprobleme der Elektronen in Metallen und Halbleitern

In general the electrical resistivity of metal specimens does not depend on their size. At low temperatures, however, the mean free path of the electrons can be comparable with the geometric dimensions of the specimens. In this paper the theory of K. Fuchs and R.B. Dingle is discussed in detail, and it is shown that there is a discrepancy between their formulae and the experimental results of van den Berg and hitherto unpublished own measurements for gold at liquid helium temperatures. On the assumption of Rutherford-scattering in a semiconductor one can see that in first approximation the mechanism of electron scattering has only little influence on the formulae derived by Fuchs for thin films. In agreement with similar observations on the anomalous skin-effect the authors propose to explain· the discrepancy by the increase of residual resistivity with cold working of the surface layer and therefore by the enlargement of the effective surface-area. As to thin wires the assumption of a "surface-resistivity" yields some approximated results in good agreement with those of Dingle. In conclusion, the authors give a report of the above discussed resistivity measurements on cylindrical specimens of Mylius-gold, perhaps the purest metal ever obtained

Verfeinerte Theorie der elektrothermischen Kälteerzeugung

Forty years ago E. Altenkirch had developed a theory concerning the use of the electrothermic Peltier effect for refrigerating purposes. This theory is too much simplified in comparison with the practical difficulties and the present authors give a more detailed calculation here. First they consider the influence of non cylindric forms of the thermoelements. Secondly the new calculations demonstrate the great influence even of small contact resistances on the maximal temperature diminuition and efficiency of electrothermic cooling plants. In conclusion the authors discuss the diminuition of the practical chances of the electrothermic cooling method from the standpoint of the new detailed theory

Standardized ultrasound evaluation of carotid stenosis for clinical trials: University of Washington Ultrasound Reading Center

<p>Abstract</p> <p>Introduction</p> <p>Serial monitoring of patients participating in clinical trials of carotid artery therapy requires noninvasive precision methods that are inexpensive, safe and widely available. Noninvasive ultrasonic duplex Doppler velocimetry provides a precision method that can be used for recruitment qualification, pre-treatment classification and post treatment surveillance for remodeling and restenosis. The University of Washington Ultrasound Reading Center (UWURC) provides a uniform examination protocol and interpretation of duplex Doppler velocity measurements.</p> <p>Methods</p> <p>Doppler waveforms from 6 locations along the common carotid and internal carotid artery path to the brain plus the external carotid and vertebral arteries on each side using a Doppler examination angle of 60 degrees are evaluated. The UWURC verifies all measurements against the images and waveforms for the database, which includes pre-procedure, post-procedure and annual follow-up examinations. Doppler angle alignment errors greater than 3 degrees and Doppler velocity measurement errors greater than 0.05 m/s are corrected.</p> <p>Results</p> <p>Angle adjusted Doppler velocity measurements produce higher values when higher Doppler examination angles are used. The definition of peak systolic velocity varies between examiners when spectral broadening due to turbulence is present. Examples of measurements are shown.</p> <p>Discussion</p> <p>Although ultrasonic duplex Doppler methods are widely used in carotid artery diagnosis, there is disagreement about how the examinations should be performed and how the results should be validated. In clinical trails, a centralized reading center can unify the methods. Because the goals of research examinations are different from those of clinical examinations, screening and diagnostic clinical examinations may require fewer velocity measurements.</p