The co-operative Jahn-Teller effect is a phase transition which is driven by the interaction between localized orbital electronic states and the crystal lattice. It involves the simultaneous splitting of the electronic states and a symmetry-lowering distortion of the lattice. Recent advances in the understanding of this effect have been spectacular both because of the discovery of a family of transparent rare earth compounds in which it is easy to measure the changes of energy of the electronic states and because of the increasing use which has been made of ultrasonic techniques for investigating lattice distortions. It will be shown that in most materials the coupling is predominantly to a strain (which changes the shape of the crystal) and the ultrasonic method provides a direct observation of the softening of the appropriate elastic constant near the transition temperature. This strain mode coupling provides an effective interaction between the Jahn-Teller active ions which has a very long range. As a result molecular field theory can be used to provide a quantitative description of very many of the properties of these systems.The article discusses in detail the possible origins, symmetries and properties of the electron-lattice interactions and how they lead to possible hamiltonians for the coupled system. The relation of these interactions with quadrupolar interactions and with magnetostriction is included. The methods of solution of the hamiltonians which are discussed lead to an understanding of the electronic states, the phonon spectrum and the mixed normal modes. A very wide variety of experimental techniques has been brought to bear on this problem. These are reviewed in detail and the results are compared with theoretical expressions whenever possible. T h e application of external stress and magnetic field turns out to be of particular significance in the case of the rare earth compounds, because here they can produce effects which are larger than the low-temperature spontaneous effects.

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A planar YBa2Cu3Ox (YBCO) coil has been used as a pick-up coil in a nuclear magnetic resonance imager. It was found that the quality factor (Q) of the YBCO coil was greater than its copper counterpart by approaching 50% even in a field of 0.15 T. The Q of the copper coil at 77 K and in a field of 0.15 T was 1200 while that of the YBCO coil under identical conditions was 1700.

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