Optimizing Thomson’s jumping ring

American Journal of Physics, 2000

Measurements of the phase delay of the current and force on a ring floated on a commonly available Thomson's jumping ring apparatus were performed for phase angles from 12°to 88°. The force and phase data show excellent agreement with a linear inductive model. We find that the demonstration, as usually performed with large highly conducting rings, operates in the inductance-dominated regime at 60-Hz line frequency. Stroboscopic photographs of the jumping ring, for both room-temperature and 78-K rings, confirm that the same time-averaged inductive phase lag mechanism, not an electrical transient, accounts for the jump height. We introduce a simple room-temperature demonstration that illustrates the importance of the phase lag: Despite its greater weight, a stack of thin rings will float higher than a single ring as the inductive phase lag comes to dominate the parallel resistance of the combined rings.

2013

The Thomson ring experiment is revisited with the aim of obtaining higher and higher jump heights. A new way to determine the inductance of the ring and the phase lag between the primary magnetic flux and the current in the ring is introduced. New more effective configurations are presented, up to that of true electromagnetic cannon that fires aluminium disks with great violence. As an interesting by-product, regular geometric patterns of the iron filings used to visualize the structure of the very high magnetic fields inside the coil of the electromagnetic cannon are revealed, indicating the emergence a self-organization phenomenon typical of dissipative systems.

American Journal of Physics, 2015

We present a new model, and the validating experiments, that unveil the rich physics behind the flight of a conductive ring in the Thomson experiment, a physics veiled by the fast thrust that impels the ring. We uncover interesting features of the electro-dynamics of the flying ring, e.g. the varying mutual inductance between ring and the thrusting electromagnet, or how to measure the ring proper magnetic field in the presence of the larger field of the electromagnet. We succeed in separating the position and time dependences of the ring variables as it travels upward in a diverging magnetic field, obtaining a comprehensive view of the ring motion. We introduce a low-cost jumping ring setup that incorporates simple innovative devices, e.g. a couple of pickup coils connected in opposition that allows us to scrutinize the ring electrodynamics, and to confirm the predictions of our theoretical model with good accuracy. This work is within the reach of senior students of science or engineering, and it can be exploited either as a teaching laboratory experiment or as an open-end project work.

2015

In this extended essay, a jumping ring setup has been established in order to find magnetic force applied on a ring as a function of applied current. A basic setup consists of a solenoid, ferromagnetic rod core and non-ferromagnetic electrically well conductive ring put on the core. In this work, however, a programmable microcontroller unit was included into the system so that time of flight of the ring could be measured. Electrical devices were also included for overcurrent protection. A set of time of flight versus jumping distance of an aluminum ring measurement has been made together with voltage and current measurements on the solenoid. Impedance of the solenoid was found using voltage-current linear curve fit. From inductance-impedance relationship, magnetic energy stored in the solenoid was calculated. Ring speeds were calculated to obtain the acceleration and hence the net force applied on the ring. Net force acting on the ring as a function of current was plotted. Finally, ...

International Journal of Material Forming, 2009

We present a mathematical method for identifying, separating and quantifying the 3D significant distortions. Measurement of a gas quenched C-ring type sample is performed by a Coordinate Measuring Machine (CMM). Quenching simulation is done with the commercial software Forge 2008 TTT®. After comparisons, we notice that the distortions and their tendencies are the same but not exactly the amplitudes. We only focus on distortions which have a physical origin, like the pincers opening, often observed in the literature. As a first explanation, we underline the role of phase transformations and volume dilatation of the steel during quenching.

Journal of the Mechanics and Physics of Solids, 2004

The electromagnetic forming (EMF) process is an attractive manufacturing technique, which uses electromagnetic (Lorentz) body forces to fabricate metallic parts. One of the many advantages of EMF is the considerable ductility increase observed in several metals, with aluminum featuring prominently among them. The quantitative explanation of this phenomenon is the primary motivation of this work. To study the ductility increase due to EMF, an aluminum ring is placed outside a ÿxed coil, which is connected to a capacitor. Upon the capacitor's discharge, the time varying current in the coil induces a larger current in the ring specimen and the resulting Lorentz forces make it expand. The coupled coil-ring electromagnetic and thermomechanical problem is solved, using an experimentally obtained constitutive model for a particular aluminum alloy. Our results show that for realistic imperfections, the EMF ring starts necking at strains about six times larger than its static counterpart, as observed experimentally. This study establishes the importance of solving the fully coupled electromagnetic and thermomechanical problem and provides insight on how di erent constitutive parameters in uence ductility in an EMF process.

SAE International Journal of Passenger Cars - Mechanical Systems, 2015

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Scripta Materialia, 1996

International Journal of Impact Engineering, 2006

High-temperature mechanical properties for metals are usually measured under quasi-isothermal conditions, where thermal equilibrium has been achieved by slowly preheating the samples to a predetermined temperature. However, there are several situations of practical interest where the material is heated rapidly, so thermal diffusion is negligible during the time of heat deposition and mechanical deformation. Hypervelocity impact and penetration, shaped charge jet formation, and pulsed heating in electromagnets are some such situations. Experimental evidence from electron beam heating indicates that high-temperature mechanical properties significantly depend on the rapidity of heat deposition. As the time duration of heating is reduced, the amount of local heat transfer decreases due to limited thermal diffusion. Thus, the thermodynamic process deviates from the isothermal process and approaches the adiabatic process for pulsed heating conditions. Therefore, it is imperative that mechanical properties be measured under appropriate thermodynamic conditions. We propose that the electromagnetically driven expanding ring experiment be used to measure the adiabatic mechanical properties of metals. While earlier expanding ring experiments were conducted primarily to obtain high-strainrate strength and fragmentation data, our primary goal is to obtain high-temperature data under pulsed heating conditions approaching the adiabatic process. Our preliminary data suggest that the adiabatic mechanical properties are quite different from isothermal properties.

2018

A fully coupled semi-analytical model is proposed for a ring expansion test. In this model, we consider the electromagnetic, mechanical and thermal effects. During the development, an ideal case of electromagnetic ring expansion test is modelled using both semi-analytical and finite element methods. The analytical model also includes the changes in electrical resistance, mutual inductance and self-inductance as a function of radius during the ring expansion. The development procedure is divided into four separate calculation parts. Each individual part is validated before making the independent validation of the semi-analytical method to obtain a model with high accuracy and a robust calculation speed. The final prediction using this model closely resemble with the coupled finite element predictions, and it can be further extended to exploit for an inverse identification problem.