13 17 has to contain at least 10 spins at a temperature of 1 oK to obtain useful results 10 11 as compared with claims of 10 to 10 spins for the modern resonance saturation techniques under equivalent circumstances. This enormous difference can be traced back to two causes: the ratio of the quality factors of the resonant cavity 4 ('" 1 0 ) and the coils used at audio and sub-audio frequencies (;S 1) and the ratio of the resonance and relaxation absorption coefficients (oc X") that are actually measured. Comparing (4. 25) with (5. 5 . . . 8) one finds that this last factor is of the orderofHIL1 H, the quotient of the external field and the linewidth of the resonance 2 3 line. This ratio is of the order 10 for concentrated samples, to 10 for diluted samples. Literature. Steady state saturation techniques. ESCHEN FELDER, A. H. , and R. T. WEIDNER: Phys. Rev. 92, 869 (1953). BOLGER, B. : Proc. Koninkl. Ned. Akad. Wetenschap B 62,348 (1959); - Thesis Leiden 1959. Pulse saturation techniques. GIORDMAINE, J. A. , L. E. ALSOP, F. R. NASH, and C. H. TOWNES: Phys. Rev. 109, 302 (1958). DAVIS jr. , F. C. , M. W. P. STRANDBERG, and R. L. KVHL: Phys. Rev. 111, 1268 (1958). BOWERS, K. D. , and W. B. MIMS: Phys. Rev. 115, 285 (1959). DREWES, G. W. J. : Thesis Leiden 1967. Other resonance techniques.

Inhalt
Paramagnetic Relaxation.- A. Introduction.- B. Spin-spin relaxation.- C. Spin-lattice relaxation.- General references.- Electron Spin Resonance.- I. General principles.- II. Parameters associated with electron resonance spectra.- III. Experimental technique.- IV. Basic absorption processes.- V. g-values and their theoretical interpretation.- VI. The Spin Hamiltonian.- VII. Hyperfine splitting and the Spin Hamiltonian.- VIII. Examples of spectrum analysis.- IX. General conclusion.- Structural Information from Paramagnetic Resonance.- A. Introduction.- B. Free radicals.- C. Triplet state systems and biradicals.- D. Defects in insulating solids.- E. Defects in semiconductors.- F. Miscellaneous topics.- General references.- Austauschwechselwirkung in Isolatoren.- Einführung.- A. Die Theorie des Wasserstoffmoleküls in der Heitler-London-Näherung.- B. Die direkte Austauschwechselwirkung.- C. Die indirekte Austauschwechselwirkung.- D. Die Austauschwechselwirkung und die Wellenfunktionen der Elektronen im Kristall.- Magnetic Semiconductors.- A. Preface.- B. Problems related to the electron band structure of magnetic materials.- C. Theory of electron transport properties.- D. Theory of indirect exchange.- E. Electron orbitals and energies in rare earth ions.- F. Optical properties of crystals.- G. Experimental evidence for indirect exchange.- H. Electrical properties of magnetic semiconductors.- Sachverzeichnis (Deutsch-Englisch).- Subject Index (English-German).