The critical shear stress z, to move a dislocation through a random array of obstacles in the glide plane is calculated using a statistical theory. The result is an expression for t C in terms of the obstacle concentration, the line tension of the dislocation, and of the interaction force between the dislocation and a single obstacle. Fleischer's solution of the same problem is not reproduced by the statistical theory. Quantitatively the two results are not very different, but our new result is supported by some recent experimental evidence. Furthermore the theory provides a definite prescription how to combine the concentrations and interaction forces of obstacles of different kinds in the expression for zc.Die kritische Schubspannung tc, die notig ist, um cine Versetzung uber cine statistische Anordnung von Hindernissen in der Gleitebene hinwegzubewegen, wird aus einer statistischen Theorie berechnet. Das Ergebnis ist cine Formel fiir t,, in der die Hindernisdichte, die Linienspannung der Versetzung und die Wechselwirkungskraft zwischen der Versetzung und einem einzelnen Hindernis als Parameter auftreten. Das Ergebnis ist verschieden von der Losung, die von Fleischer fur dasselbe Problem angegeben wurde. Der quantitative Unterschied ist nicht grol3, aber die Analyse neuerer Experimente spricht mehr fur das Ergebnis der statistischen Theorie. Daruberhinaus liefert die Theorie eine eindeutige Vorschrift, wie die Iionzentrationen und Wechselwirkungsstarken von verschiedenen gleichzeitig vorhandenen Arten von Hindernissen, in der Formel fur tc zu kombinieren sind.

We have made measurements of the evanescent decay of the irreversible magnetization induced by magnetic cycling of solid superconducting cylinders in order to elucidate the mechanisms of Anderson's thermally activated flux-creep process. A superconducting quantum interferometer device coupled to the creep specimen by a superconducting flux transformer made possible observations of flux changes with a resolution of one part in 10 9 . The general applicability of Anderson's theory of flux creep was confirmed and the results were analyzed to show that: (1) The total flux in the specimen changed logarithmically in time, i.e., Acj> /d Int is proportional to the critical current density J c and to the cube of the specimen radius. (3) The logarithmic creep rate appears to be only weakly temperature-dependent because a proportionality to T is nearly compensated by the proportionality to J c which decreases as T increases. (4) The creep process is a bulk process that is not surface-limited (in this case). (5) Flux enters and leaves the surface in discrete events containing from about one flux quantum up to at least 10 3 flux quanta. (6) On departing from the critical state to a subcritical condition, the creep process tends to remain logarithmic in time, but the rate is decreased exponentially by decreasing T and is decreased extremely rapidly by backing off of the applied field from the critical state. (7) At magnetic fields H

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The elastic constants of the fluxoid lattice are calculated in terms of the reversible magnetisation curve by taking the appropriate derivatives of the free energy of the fluxoid assembly. The expressions obtained in previous work for the compressional modulus and for the shear modulus cq., in magnetic fields well below Hc2 are confirmed and their validity over the entire range of field between H c l and H c 2 is established. A new expression is derived for the shear modulus c6& in high fields. It turns out that cB6 goes quadratically to zero if the applied field approaches H c 2 .

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