Donald Sullivan

Low temperature direct current comparators

This paper presents an improved version of the low temperature direct current comparator described originally by Harvey. The comparator described herein exhibits a resolution of 1.5 nA ·turn and provides ratios which have uncertainties of less than 4 parts in 1010. An alternate approach to the shielding of the windings is discussed and a preliminary comparator based on this concept is shown to have equally good performance. The latter comparator is probably better suited to the attainment of large current ratios.

The NIST automated computer time service

The NIST Automated Computer Time Service (ACTS) is a telephone time service designed to provide computers with telephone access to time generated by the National Institute of Standards and Technology at accuracies approaching 1 ms. Features of the service include automated estimation by the transmitter of the telephone-line delay, advanced alert for changes to and from daylight saving time, and advanced notice of insertion of leap seconds. The ASCII-character time code operates with most standard modems and computer systems. The system can be used to set computer clocks and simple hardware can also be developed to set non-computer clock systems.

Mechanical Analogs of Time Dependent Josephson Phenomena

The pendulum analog of a small-area Josephson junction between two superconductors is coupled to the analogs of other circuit elements to demonstrate a variety of time dependent phenomena observed with actual devices. The analogy between electrical circuit and mechanical device is established through a Lagrangian formalism. The analog provides a simple and quick means of developing physical insight into this highly nonlinear system. To demonstrate the value of the analog we consider, as one example, the problem of a junction coupled to a resonant cavity and present a result that was missed by a previous solution.

A Low-Temperature Direct-Current Comparator Bridge

The application of superconducting direct-current comparators to the measurement of resistance ratios is described. One comparator consists of a binary set of ratios between 1:1 and 160:1 providing for self-calibration by a buildup procedure. A second comparator exhibiting discrete ratios of 1:1, 10:1, and 100:1 is also described. Ratio uncertainty of less than 1 part in 109 is achieved by enclosing the ratio windings in overlapping toroidal superconducting shields. Superconducting quantum interference devices (SQUIDs) serve as flux sensors for the comparators. One of these current comparators is used to calibrate a 100-?:1-? resistive divider, which at a current of 10 mA exhibits a self-heating error of 0.0023 ppm.

Can superconductivity contribute to the determination of the absolute ampere

The absolute ampere is shown to be derivable from a static levitation of a superconducting mass. The magnetic force which balances the gravitational force can be obtained from a combination of inductance and linear position measurements. A unique feature of the concept is the use of the calculable capacitor for the inductance measurements. Besides the possibility of this specific approach other concepts involving superconductivity are also discussed.

Low Temperature Voltage Divider and Null Detector

This paper describes a low temperature voltage divider and null detector which are designed to accurately bring the low level voltage of the Josephson steps to the 1 V level. The divider uses the series‐parallel interchange of resistors. Preliminary tests indicate that the resistors are stable to within one part in 107 and that the ratio is accurate to within one part in 106. The null detector utilizes a superconducting quantum interference magnetometer and is limited only by the Johnson noise in the cooled resistors.

High‐frequency limitations of the double‐junction SQUID amplifier

The double‐junction SQUID is viewed as a parametric amplifier and mechanisms, which place an upper limit on the pump frequency (and thus gain), are investigated. Self‐induced steps in the I‐V characteristics as well as damping of the Josephson oscillation are shown to be two limiting mechanisms.

Cryogenic direct current comparators and their applications

Cryogenic direct current comparators, utilizing superconducting shields and Superconducting Quantum Interference Devices (SQUIDs), provide current ratios of up to 100/1 or higher with accuracies of \lsim 1 \times 10^{-9} and current resolutions of \lsim 6 \times 10^{-11} A. Two types of comparators differing in the shielding arrangement of the ratio windings are described. One type consists of unit windings inside a seamless Pb tube; the other consists of multi-turn ratio windings within an overlapping toroidal superconducting shield. For both shielding configurations, SQUIDs serve as flux sensors for the comparators. The application of these comparators to the measurment of resistance ratios is described.

Resistance of a Silicon Bronze at Low Temperatures

A specially prepared silicon bronze is shown to be quite insensitive to magnetic field and temperature change at liquid helium temperatures. The temperature coefficient of resistance is of the order of a few parts per million per degree over the range 2–10 K and the change in resistance at a flux density of 1 T (104 G) is about 10 ppm.

Time generation and distribution

The authors present a broad overview of time and frequency technology, particularly those trends relating to the generation and distribution of time and frequency signals. The authors provide a general look at these trends. They refer the reader to other papers, particularly to those in this issue, for greater detail. The topics considered are: background, including accuracy and stability, frequency standards, time transfer systems, and network synchronization; time generation, including various types of atomic clocks and quartz oscillators; and time distribution, including applications of time transfer concepts and practical synchronization limits. The characterization of components and systems is also addressed. >