932J. ELMER RHODES, JR. magnetization, and, therefore, in resistivity, it was necessary to separate this effect from the change in restivity due to domain orientation. Th£ separation was made by the method used by Englert, 19 by extrapolating the p vs. H curve, as illustrated in Fig. 12 for S3 percent nickel. The points for transverse magnetization lie on a straight line permitting easy extrapolation to the demagnetizing field corresponding to saturation; for the specimen of tape used this was small, (N/4:ir)B 8 = 0.047X9800 = 450. The same slope of p vs. H curve was used to extrapolate the longitudinal measurements to H=0.The velocity of sound in parahydrogen, normal hydrogen, and 50 percent para-50 percent orthohydrogen mixtures has been observed at several temperatures, and over a range of frequency to pressure ratio from one to 60 megacycles per atmosphere. Dispersion attributed to failure of the rotational degrees of freedom to follow the temperature associated with the translational degrees of freedom has been observed with all mixtures and at every temperature at which observations were made. Measurements at two different frequencies for similar samples at the same temperature indicate that frequency and pressure affect the velocity of sound only as the quotient, frequency/pressure, with the exception of small corrections C HANGES of sound velocity in some gases with variation of the frequency to pressure ratio have been known for a number of years. These have been attributed to failure of the internal degrees of freedom of the molecules to follow temperature changes in the sound wave. The first phenomenon of this type to be observed oersteds. The results are probably not accurate to less than 0.1 percent, but appear to lie on a smooth curve. The explanation of the shape of the curve is not apparent. On the one hand one might expect the effect to be large when the curie point is low, as it is at each end of the range of alloys studied, because there the change in spontaneous magnetization with field is relatively large. On the other hand, near 70 to 75 percent nickel the crystal anisotropy is zero, and order-disorder phenomena are observed, and there may be some connection, not now established, between these and the effect of spontaneous magnetization on resistivity.I am indebted to Dr. C. L. Dolph and Miss C. L. Froelich for assistance with the mathematics and computations involved in the solution of the energy equation. that must be applied because hydrogen is not a perfect gas. Experiments indicate that the dispersion occupies a greater range of frequency to pressure than would be expected if the rotational specific heat behaved as a simple relaxation phenomenon. It is shown that a simple relaxation phenomenon is not to be expected, and that the dispersion in parahydrogen can be characterized, approximately, by two relaxation frequency to pressure ratios, one for the rotational transition 0-2, and another for the transition 2-4. An expression for these relaxation frequency to pressure ratios, that rough...

As a belt is taken up on a pulley, a varying tension will be laid down, or “recorded,” as a strain pattern. As the belt pays off, the pattern “plays back” creating a corresponding varying tension on the next span. If the delay is proper, the oscillating tension associated with transverse vibration of a belt span can feed around the belt circuit and parametrically drive the vibration. A belt span may be self-excited into vibration by this mechanism without imperfections in either the belt or pulleys.

The concept of radiation pressure exerted against a reflecting surface by electromagnetic waves and by sound waves in gases is a familiar one. The absence of radiation pressure for sound waves in a “linear medium” is becoming well known. The equations are somewhat different for the radiation pressure for these two cases where the radiation pressure is not zero. The mechanisms usually called on to explain them (nonlinearity of the equation of state of the gas, acceleration of charges in the reflector by the electromagnetic waves) are widely removed from each other. Consequently, it is not evident on the surface whether radiation pressure is a general property of waves or the capricious result of isolated physical phenomena associated with some waves. A generalized concept of radiation pressure is here developed and then applied to several kinds of waves: sound waves in various media, waves on strings and springs, electromagnetic waves. Also, a simple demonstration of radiation pressure (for waves in a ripple tray) is described.

The formation of an optical image by a lens is often treated so that the image appears as the double Fourier transform of the object, limited by the aperture of the lens and other stops. This treatment is here presented including details that are usually omitted in articles where an application of the treatment is the primary purpose. Separate developments are presented that are most adaptable, respectively, to the configuration usually used with a microscope objective with a short object distance and a large image distance, and to the configuration normally used with a telescope objective.

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