Volume 8, Issue 5, 01 May 1937
Index of content:
8(1937); http://dx.doi.org/10.1063/1.1710305View Description Hide Description
Energy losses in ferromagnetic materials subject to alternating fields have long been considered as due solely to hysteresis and eddy currents. However, at the low flux densities encountered in certain communication apparatus, a further loss is observed which has been variously termed ``residual,'' ``magnetic viscosity,'' or ``square law hysteresis.'' The search for an explanation of this loss has led to precise measurements of hysteresis loops with a vacuum ballistic galvanometer, and of a.c. losses with inductance bridges. From these results, it appears that that part of the a.c. effective resistance of a coil on a ferromagnetic core which is proportional to the coil current is strictly identified with the hysteresis loop area as measured by a ballistic galvanometer, or as indicated by harmonic generation in the coil. The hysteresis loop can now be constructed in detail as to size and skewness on the basis of a.c. bridge measurements. This conclusion was reached previously on a compressed iron powder core, and is now confirmed on an annealed laminated 35 permalloy core. Observed eddy current losses for this core exceed those calculated from classical theory by 20 percent. This excess is ascribed to the presence of low permeability surface layers on the sheet magnetic material. The a.c. residual loss per cycle (nominally independent of frequency, like hysteresis) is not observed by ballistic galvanometer measurements, although it indicates an energy loss some eight times the hysteresis loss for the smallest loop measured (Bm = 1.3 gauss). Analysis of the residual loss shows that it increases with frequency up to about 500 cycles, and remains constant at higher frequencies (to 10,000 cycles per second). Concurrently with the increase of residual loss, the permeability of the alloy is observed to decline with increasing frequency about 1 percent below the value predicted from eddy current shielding. This effect is most noticeable at frequencies below 1000 cycles.
8(1937); http://dx.doi.org/10.1063/1.1710306View Description Hide Description
This paper describes a theoretical and experimental study of heat effects in viscous liquids up to rates of shear of the order of a million reciprocal seconds, corresponding to the rates experienced in lubrication practice. Even at much lower rates the effect of heat generated by viscous resistance in a capillary tube is sufficient to produce an appreciable drop in the apparent viscosity. This drop cannot be fully corrected for by laboratory observations of the mean efflux temperature, since the temperature rise will not be uniform over the cross section. Detailed calculations are necessary in order to distinguish between heat effects and non‐Newtonian behavior. The theory is illustrated by experimental data on lubricating oils.
8(1937); http://dx.doi.org/10.1063/1.1710308View Description Hide Description
The viscosities of three lubricating oils have been investigated at 100°, 130°, 210.2°F at pressures ranging from atmospheric to 4000 atmospheres (57,000 lb./in.2). While the oils were from fields widely separated geographically, their initial viscosities were matched at 0.4 poise at 130°F. At a pressure of 26,000 lb./in.2 at 130°, however, the viscosities were strikingly different; the viscosity of the Pennsylvania oil increased 25‐fold, and the Oklahoma oil 35‐fold, but for the California sample the increase of viscosity was greater than 100‐fold. The effect of pressure on the temperature coefficient of viscosity and the effect of temperature on the pressure coefficient of viscosity are discussed.