The variation with temperature of the penetration depth of weak magnetic fields into niobium has been measured. The variation was more rapid than expected from the BCS theory of superconductivity, in contrast to the situation in previously measured superconductors where it was less rapid. Just as in the previous cases, the results here can be understood in terms of a variation of the energy gap different from that predicted by the BCS theory. A comparison with the energy gap deduced by Dobbs and Perz from their ultrasonic-attenuation measurements is given. The penetration depth at absolute zero, \(0), is estimated from the present results to be 470=1=50 A, while the London penetration depth XL(0) is 390=b50 A.

Long-range, torsional guided waves generated in pipes using magnetostrictive sensors (MsSs) have great potential for applications to the structural health monitoring (SHM) of hard-to-inspect pipes. This paper reports an improved MsS technique (when compared to related techniques currently used for the NDT of pipes) that uses polymeric magnetic tape material that is suitable for use in a variety of industries as an SHM tool for pipes. Improvements include increased efficiency, reduced cost and increased long-term survivability of the sensor system. Transduction efficiency was increased by reducing the sensor eddy current losses and by using a field concentrator strip. For long-term monitoring, a low-cost magnetic oxide based MsS material (video recording tape) having the required magnetic properties was used. The MsS strips were oriented to generate non-dispersive torsional guided ultrasonic waves that propagate long distances with minimal mode conversion. Further, considering both safety and long-term survivability of the sensor, low-power ultrasonic instrumentation was developed and tested. Measurements reported here demonstrate the sensitivity of this sensor to both radial notches (saw cuts) and drilled holes. Results also show that magnetic anisotropy of the strip plays a role in generating torsional waves. It is envisioned that results obtained from the present study will significantly enhance the ability to monitor the long-term structural health of piping systems.

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The reversible folding of cytochrome c in urea at pH 4.0 was investigated by repetitive pressure perturbation kinetics and by equilibrium spectroscopic methods. Two folding reactions were observed in the 1 ms to 10 s time range. The rates and amplitudes of these reactions depend on urea concentration in a complex manner, which is different for each process. The absorbance spectra of the kinetic amplitudes of the two reactions also differ from each other. A model with a three-state mechanism can quantitatively account for all of the kinetic and equilibrium data, and it enables us to determine the rate constants and volume changes of the two steps. If a rapid protonation step is added to the mechanism, the analysis can be extended to calculate the pH dependence of the rate and amplitude of the faster folding step. This pH dependence is in excellent agreement with previously published data [Tsong, T. Y. (1977) J. Biol. Chem. 252, 8778-8780]. Kinetic experiments in the 695-nm band show clearly that the axial ligand methionine-80 is involved in the slow folding process and the other axial ligand, histidine-18, is involved in the fast process. Additional experiments with a cyanogen bromide fragment of the protein, and fluorescence detection of the folding kinetics of the intact protein, support an interpretation of the model in terms of known structural elements of cytochrome c. This work provides new information about the mechanism of the folding of cytochrome c, resolves conflicts in earlier interpretations, and demonstrates the applicability of the repetitive pressure perturbation kinetics method to protein folding.

The construction and operation of a repetitive pressure perturbation apparatus to study rapid chemical relaxation kinetics is reported. The reaction progress is monitored by measuring light absorption, light scattering, or fluorescence. This new instrument allows the application of small pressure perturbations (0.01–5.0 atm), which is a feature of special interest when studying reactions that are sensitive to small changes in free energy (e.g., quasi-phase changes in biomembranes and conformational changes in macromolecules). Repetitive application of the pressure perturbation and averaging of consecutive measurements greatly enhances the sensitivity and allows the use of such small pressure perturbations. The time range available for kinetic studies is 20 μsec to several minutes. Examples of the performance of the new apparatus are given, and the theoretical analysis is discussed insofar as it differs from conventional relaxation techniques.

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