71 For a given value of I the field is independent of the geometrical composition of the coil inside the winding space. The actual number of turns and the cross­ section of the conductors is entirely determined by the impedance of the power supply to which the magnet should be adapted. In the case of low impedance (high current and low voltage) few turns of thick metal should be used. In the case of high impedance (low current and high voltage) many turns of thin material are needed. High impedance coils are made of square wire or flat strip wound into layers or "pancakes" 1. A nice system for low impedance coils was deve­ loped by BITTER. The turns of his magnets consist of flat copper discs separated by thin insulating sheets and joined together at their edges. In this type of coil the current density is higher near the axis than at the exterior, resulting into a higher value for G (see above). For the details of the construction we refer to the original papers 2, 3. If the power is dissipated at a low voltage the cooling may be achieved with the help of water. Distilled water should be preferred over mains' water in order to prevent the magnet from corrosion. In the case of a high voltage coil some non-inflammable organic fluid should be used. A low viscosity and a large specific heat are advantageous.

Inhalt

Low Temperature Magnetism.- I. Introduction.- II. Effects of magnetic and of electric fields on the energy levels of the magnetic ions.- III. Older research methods.- a) Measurements of the paramagnetic susceptibilities.- b) The influence of magnetic and electric fields on the spectra.- e) The Faraday effect.- IV. Paramagnetic relaxation.- V. Paramagnetic resonance.- VI. Antiferromagnetism.- Adiabatic Demagnetization.- A. Fundamental considerations.- I. Introduction.- II. Thermodynamics of the demagnetization process.- III. Absolute temperature determination.- B. Experimental methods.- I. Introduction.- II. Demagnetization Cryostats.- III. Magnets.- IV. Bridge methods.- C. Magnetic investigations at relatively high temperatures.- I. Theoretical considerations.- II. Results obtained with individual salts.- III. The influence of magnetic fields.- D. Magnetic investigations at the lowest temperatures.- I. Cooperative effects.- II. Results obtained with invidual salts.- III. The influence of magnetic fields.- E. Other investigations below 1° K.- I. Heat transfer and thermal equilibrium.- II. Experimental results.- III. The thermal valve and its applications.- IV. Nuclear demagnetization and nuclear orientation.- General references.- Superconductivity. Experimental Part.- I. Introductory survey.- II. Electrical and magnetic properties of macroscopic superconductors.- III. Thermodynamic properties of the normal and superconductive phases.- IV. Penetration of a magnetic field into a superconductor.- V. Phenomena associated with the surface energy between the superconductive and normal phases.- VI. Thermal effects.- VII. Superconductive alloys and compounds.- VIII. Diverse properties unchanged in the superconductive transition.- References Appended in Proof, March 1956.- Theory of Superconductivity.- I. Introduction.- II. Thermodynamic properties and two-fluid models.- a) Thermodynamic relations.- b) Two-fluid models.- III. London theory and generalization.- a) London theory.- b) Solutions of the London equations.- c) The London approach to superconductivity.- d) Pippard's non-local modification of the London equation.- e) Derivation of diamagnetic properties from energy gap model.- f) Non-local theories.- IV. Boundary effects; the intermediate state.- a) Theory of boundary energies.- b) Applications to specific problems.- c) The intermediate state.- V. Electron-phonon interactions.- a) Introduction.- b) Formulation of the electron-phonon interaction problem.- c) Calculation of interaction energy.- General references.- Liquid Helium.- A. Historical survey.- B. The diagram of state.- C. Entropy.- D. Superfluidity.- E. Viscosity.- F. Heat conduction.- G. Wave propagation.- H. The saturated film.- J. The unsaturated film.- K. Theoretical Appendix.- Literature references.- Sachverzeichnis (Deutsch-Englisch).- Subject Index (English-German).