Use of Ring-Disk Equipment in Coulometry

Abstract

We have found it difficult to implement coulometry on small volumes and yet react the sample quantitatively in a reasonable time. We describe here a simple and satisfactory solution, making use of a commercial ring-disk electrode system.¹ A small cup-like cell is constructed with dimensions such that the ring-disk assembly can rotate freely inside it at a small distance from the bottom, as shown in the figure. The ring is used as the counter-electrode, and appears to be acceptable in spite of a much smaller area than the working electrode (the disk). The reference electrode is provided by a capillary connection at the bottom. The capillary is filled with a solution of supporting electrolyte and agar, which in turn is connected through a salt-bridge (containing electrolyte only) to a calomel electrode. The level of the electrolyte in the reference arm is higher than that in the cell, so that a very small positive flow is ensured to prevent contamination of the agar with the sample. The very smooth rotation of the ring-disk system permits speeds up to about 2000 RPM. It was found that for electrolysis of metals such as Cd and Pb on a mercury-coated glassy carbon electrode, the half-time for electrolysis is about 2.5 min at the optimum rotation speed of 800 RPM, with a sample volume of 10 μL.     

Evaluation of production costs of two interconnected electric power systems by the segmentation method

This paper develops the methodology, based on the segmentation method, to evaluate the production costs of two interconnected systems. The demand plane is divided into a grid structure, or segments, of equal size. Each segment contains the joint probability of the occurrence of load in the range of the selected segment as well as the first moments, or expected values, of load, or equivalent load, and residual tie line capacity for each system. The methodology proposed is accurate and computationally efficient. The method is applied to the evaluation of expected energy generation, unserved energy as well as production costs of two interconnected systems. The impact of tie line capacity variations, forced outage rate changes as well as joint ownership of generation is investigated.     

Connectivity requirements for Byzantine agreement under restricted types of failures

We investigate the problem of reaching Byzantine Agreement in arbitrary networks where both processors and communication links are subject to omission or stopping faults. For the case of deterministic, synchronous algorithms we give a necessary and sufficient condition relating the solvability of the problem to the connectivity of the network. In particular, we show that an algorithm resilient to at mostt faulty processors andk faulty links subject to omission or stopping faults exist, if and only if the network has a connectivity pair (t, k)>(t, k).Vassos Hadzilacos received his BSE from Princeton in 1980 and his PhD from Harvard in 1984, both in Computer Science. He is presently an Assistant Professor at University of Toronto. His research interests are synchronisation and reliability in distributed computing. He is a co-author of a book on Concurrency Control and Reliability in Database Systems.     

On the message complexity of binary byzantine agreement under crash failures

Summary The binary Byzantine Agreement problem requiresn–1 receivers to agree on the binary value broadcast by a sender even when some of thesen processes may be faulty. We investigate the message complexity of protocols that solve this problem in the case of crash failures. In particular, we derive matching upper and lower bounds on the total, worst and average case number of meassages needed in the failure-free executions of such protocols.More specifically, we prove that any protocol that tolerates up tot faulty processes requires a total of at leastn+t–1 messages in its failure-free executions —and, therefore, at least [(n+t–1)/2] messages in the worst case and min (P ₀,P ₁)·(n+t–1) meassages in the average case, whereP _v is the probability that the value of the bit that the sender wants to broadcast isv. We also give protocols that solve the problem using only the minimum number of meassages for these three complexity measures. These protocols can be implemented by using 1-bit messages. Since a lower bound on the number of messages is also a lower bound on the number of meassage bits, this means that the above tight bounds on the number of messages are also tight bounds on the number of meassage bits. Vassos Hadzilacos received a BSE from Princeton University in 1980 and a PhD from Harvard University in 1984, both in Computer Science. In 1984 he joined the Department of Computer Science at the University of Toronto where he is currently an Associate Professor. In 1990–1991 he was visiting Associate Professor in the Department of Computer Science at Cornell University. His research interests are in the theory of distributed systems. Eugene Amdur obtained a B. Math from the University of Waterloo in 1986 and a M.Sc. from the University of Toronto in 1988. He is currently employed by the Vision and Robotics group at the University of Toronto in both technical and research capacities. His current areas of interest are vision, robotics, and networking. Samuel Weber received his B.Sc. in Mathematics and Computer Science and his M.Sc. in Computer Science from the University of Toronto. Currently, he is at Cornell University as a Ph.D. student in Computer Science with a minor in Psychology. His research interests include distributed systems, and the semantics of programming languages.     

Single-stage surgery for implant placement: a retrospective study

Single-stage implant placement surgery offers patients a number of advantages. These include reduced cost and time commitments and greater comfort, function, and convenience. This article reports on the results of using a single-stage technique with 58 patients fitted with 240 implants. No implant failures were detected in any patients prior to the final attachment of a Dolder bar. Among the 27 patients and the 125 implants followed for a longer period of time, 99.2% exhibited no signs of implant mobility or pain. Indicators of gingival health and oral hygiene ranged in quality from acceptable to ideal.     

RMR-efficient implementations of comparison primitives using read and write operations

We consider asynchronous multiprocessors where processes communicate only by reading or writing shared memory. We show how to implement consensus, compare-and-swap and other comparison primitives, as well as load-linked/store-conditional (LL/SC) using only a constant number of remote memory references (RMRs), in both the cache-coherent and the distributed-shared-memory models of such multiprocessors. Our implementations are blocking, rather than wait-free: they ensure progress provided all processes that invoke the implemented primitive are live. Our results imply that any algorithm using read and write operations, and either comparison primitives or LL/SC, can be simulated by an algorithm that uses read and write operations only, with at most a constant-factor increase in RMR complexity.     

On the power of shared object types to implement one-resilient Consensus

Abstract. In this paper we study the ability of shared object types to implement Consensus in asynchronous shared-memory systems where at most one process may crash. More specifically, we consider the following question: Let and be a set of object types that can be used to solve one-resilient Consensus among n processes. Can always be used to solve one-resilient Consensus among n - 1 processes? We prove that for n = 3 the answer is negative, even if consists only ofdeterministic types. (This strengthens an earlier result by the first author proving the same fact for nondeterministic types.) We also prove that, in contrast, for the answer to the above question is affirmative. Received: July 1997 / Accepted: May 2000