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.