Peter Graneau

First indication of ampere tension in solid electric conductors

Abstract An empirical law for the mechanical force between two current-elements, originally deduced by Ampere from a series of classical experiments, asserts that an electric current flowing along a straight wire should place the wire in tension. The existence of longitudinal Ampere forces at solid-liquid conductor interfaces has been demonstrated by various investigators during the past 160 years. This letter contains the first report of pulse currents creating sufficient tension to cause fracture in hot copper and aluminum wires.

Ampere tension in electric conductors

It is shown that the application of the Ampere and Lorentz force laws to a closed current in a metallic circuit results in two different mechanical force distributions around the circuit. In addition to the transverse forces, which both laws predict, the Ampere electrodynamics requires a set of longitudinal forces that subject the conductor to tension. These longitudinal forces explain electromagnetic jet propulsion and the recoil mechanism in a railgun. Pulse current experiments are described in which Ampere tension shattered solid aluminum wires. Electrons moving through the metal lattice are the basic current-elements of the Lorentz force theory. But Ampere assumed his current-elements to be infinitely divisible. With the help of computer-aided analysis and experiment, it is demonstrated that the amperian current-element must also be of finite size and involve at least one lattice ion in addition to the conduction electron. Calculations with Amperes formula have been found to give reasonable results when the atom, or unit atomic cell, is taken to be the smallest possible current-element. Some technological consequences of Ampere tension are discussed briefly with regard to pulse currents in normal conductors and steady currents in superconductors. The use of large macroscopic current-elements of unit length-to-width ratio gives rough approximations to the Ampere tension. The accuracy of the calculations can be improved by resolving the conductor into a number of parallel filaments, each filament being subdivided into cubic current-elements.

Application of Ampere’s force law to railgun accelerators

This paper examines the question of where in the railgun accelerator circuit is the seat of the recoil force. It is explained that conventional electromagnetic field theory and the older Ampere electrodynamics disagree on this point. The former places the recoil force in the remote gun breach while the latter claims it resides in the railheads close to the projectile. An experiment is described which tends to confirm the Ampere prediction. The second part of the paper deals with the force distribution along the projectile branch of the accelerator. Finite current‐element analysis has been employed to show that both theories give approximately the same acceleration force distribution and that the total acceleration force furnished by them agrees well with an experimental check. However, according to the Ampere law, the projectile branch of the circuit should also be subject to strut compression and not only to transverse acceleration. This aspect of the Ampere electrodynamics still awaits experimental conf...

Longitudinal magnet forces

The Ampere electrodynamics of metallic conductors and experiments supporting it predict that the interaction of a current‐carrying wire with its own magnetic field should produce longitudinal mechanical forces in the conductor, existing in addition to the transverse Lorentz forces. The longitudinal forces should stretch the conductor and have been referred to as Ampere tension. In 1964 it was discovered that a current pulse would break a straight copper wire into many fragments without visible melting. A metallurgical examination of the pieces confirmed that the metal parted in the solid state. The same observation has now been made with aluminum wires. In the latest experiments the wire was bent into a semicircle and arc‐connected to a capacitor discharge circuit. The arc connections ruled out rupture by Lorentz hoop tension and indicated that longitudinal forces may also arise in circular magnet windings. Explanations of wire fragmentation by thermal shock, longitudinal stress waves, Lorentz pinch‐off, ...

Voltage Surge Performance of Vacuum-Insulated Cryo-Cable

This paper describes the results obtained from two short experimental lengths of single-phase vacuum-insulated cables, one of which can be cooled by liquid nitrogen. Using a variety of high-voltage sources, it has been shown that there is little correlation between the energy of the source and the damage produced by sparks. Hence, the chances seem good that a cryo-cable connected to a bulk power transmission system will not suffer catastrophically with each internal spark.

Insulator flashover mechanism in vacuum insulated cryocables

This paper describes research on vacuum insulated cryocables for underground power transmission. It deals particularly with the flashover mechanism observed on solid dielectric conductor supports in vacuum. The insulators were protected by ion shields. The work revealed the important role played by gas and vapor absorbed on dielectric surfaces bridging the high voltage gap. Photographic evidence proved that an early link in the chain of breakdown events is a gas discharge (short mean free path) in the absorbed layer. Gas layer breakdown also offers explanations for the beneficial effects of ion shields with regard to breakdown voltage and protection against arc damage. With the help of high voltage capacitors it has been established that ion shielded, inorganic insulators will survive the discharge of all electrostatically stored energy in a long cable. This is an important requirement for high voltage conditioning. The paper contains the first published photograph of a Lichtenberg discharge in vacuum.

Vacuum Insulation for Cryocables and Its Resistance to Discharges

A test length of a liquid-nitrogen-cooled vacuum-insulated coaxial cable for use in underground power transmission has been studied with respect to its high-voltage insulation properties. The critical element in the cable is the spacer between the conductors, since electrical discharges in the system are easily initiated across this spacer. Previous experiments with voltage sources capable of supplying low currents indicated insignificant discharge damage to the spacer as long as inorganic materials were used. The new results given in this paper show that insulator damage may still be small for discharges with peak current amplitudes up to 40 kA. The discharge current was supplied by an energy storage capacitor bank for a large number of tests with the same spacer. In addition to current measurements, photographic records of the discharges are presented, and an assessment of the damage to the insulator and conductors is given.

Current characteristics of a high voltage 60 Hz crossed-field discharge

This paper deals with the continuation of research on vacuum pumping with 60 Hz ac voltages and permanent magnets arranged to form a crossed‐field configuration. It is well known that dc voltages in a crossed‐field discharge and in the presence of a titanium cathode will cause a decrease in residual gas pressure. In our previous research it was demonstrated that this result could also be achieved with 60 Hz ac voltages. Before applying this knowledge to commercial vacuum pumps it is important to gain an understanding of the underlying physical mechanisms. Of particular interest are the initiation and maintenance of the discharge under ac conditions. Results reported in this paper deal with a radial magnetic field in the butt‐gap between parallel disk electrodes. In this particular crossed‐field arrangement, it was found that the electron space charge is unstable, but ion production is more or less continuous with both dc and ac excitation. The pumping yield per ion was found to be low, but could probably ...

Lichtenberg Figures Produced by High-Voltage Discharges in Vacuum

The formation of Lichtenberg figures on aluminum and titanium electrodes covered by oxide and nitride layers while being exposed to high vacuum and 100-kV (rms) 60-Hz voltage is recorded. The observations were made in the course of research on high-voltage vacuum insulation for cryocables. The Lichtenberg figures have several technological implications. They provide evidence for the occurrence of 1) focused ion beams, and 2) tangential discharges in the adsorbed layer of gas on the vacuum side of dielectric coverings on metal conductors. As Lichtenberg figures can only be produced by fast high-voltage pulses, they prove that the discharge duration was short compared with the period of the 60-Hz voltage wave.

Magnetic fields in vacuum insulated high voltage power apparatus

Magnetic fields in high voltage vacuum insulation are of interest because they may produce cross-field discharges and propel vacuum arcs. This paper shows that the coaxial cable geometry with elbows and high voltage terminations permits neither the gyration of electrons nor the sufficient elongation of electron trajectories to produce a noticeable number of ionizing collisions with residual gas molecules. Experiments with a 10 toot long, 5.75 inch diameter coaxial line did not reveal the theoretically expected are motion away from the current source nor any tendency towards retrograde motion. In a butt-gap geometry, however, the kink instability of the plasma column was found to drive the are rapidly to the electrode edge. A radial magnetic field in the butt-gap enhanced are diffusion and cathode spot tracks indicated a tendency of the plasma to rotate in accordance with the Lorentz force. The beginnings of the new concept of self-sustained ion pumping are outlined. Apparatus for demonstrating this effect utilized permanent bar magnets, titanium cathodes and 60 Hz high voltage.