Volume 8, Issue 2, 01 February 1937
Index of content:
8(1937); http://dx.doi.org/10.1063/1.1710263View Description Hide Description
8(1937); http://dx.doi.org/10.1063/1.1710264View Description Hide Description
The effect of pressure on the modulus of rigidity has been determined for five metals and two glasses, to 4000 kg/cm2, by a dynamical method. The corrections due to the viscous motion of the pressure medium are computed approximately and shown to be negligible. Absolute values of the dynamical Young's modulus and modulus of rigidity are also given, and the derived compressibilities compared with directly measured compressibilities.
8(1937); http://dx.doi.org/10.1063/1.1710265View Description Hide Description
Under certain conditions, an electrodeposit from an aqueous electrolyte containing essentially nickel ammonium sulphate and sodium thiosulphate has an amorphous crystal structure and contains approximately 13 percent S and 87 percent Ni. It is shown that if this deposit is warmed to about 170°C its electrical resistance suddenly decreases and its structure changes to crystalline, without appreciable change in weight.
A Method for Measuring the Rheological Properties of Materials of Great Consistency Such as Asphalts8(1937); http://dx.doi.org/10.1063/1.1710266View Description Hide Description
It has been found that when an ASTM penetrometer needle is allowed to sink into a material such as asphalt for a series of successive time intervals, the rate of flow varies linearly with the shearing stress as calculated from these equations:,when a =rate of flow in cm3/sec.,
b =shearing stress in dynes/cm2,
Pf =final penetration during interval t, in decimillimeters,
Pi =initial penetration during interval t, in decimms.,
t =time interval in seconds,
W =weight of needle and instrument head in grams.
The first penetration in hard materials and the first several penetrations in soft materials give erratic data; otherwise no systematic errors have been observed, and accuracy depends upon the skill of the operator. As expected from plastometric theory and from previous work by other investigators: (1) Results are almost independent of the shearing stress used; (2) The mobility is affected by the shape of the surface of the material at the point of punch, while the yield value is unaffected; (3) Minus log m, which is mathematically equal to log viscosity when f=0, varies linearly with the temperature. (4) Yield values for asphalts from a particular crude stock vary linearly with the percent asphaltene content—the concentration of zero yield value being at 21.8 percent. (5) On aging, an asphalt increases in yield value and decreases in mobility. The author finds that on air‐blowing two particular charge stocks, the melting point can be predicted from the mobility at 77 degrees F:,when r and s are constants characteristic of the charge stock.
8(1937); http://dx.doi.org/10.1063/1.1710267View Description Hide Description
Specimens of each of two vulcanized rubber compounds of known composition were subjected to various constant gravity loads in a controlled temperature chamber for several months. The lengths of the stretched bands were observed from time to time while the temperature was altered in gradual steps within a range of about thirty degrees centigrade centering around room temperature. A projection method for observing small changes in length was devised for the experiment. The contraction of stretched rubber with rise in temperature, first observed about the middle of the eighteenth century, was verified, as also was its positive elongation when unstretched or when but slightly stretched. The present experiment shows that the expansion or contraction for each specimen was constant when measured in cm per degree temperature change, but not when measured in percent of the prevailing length of the band. That is, the thermal elongation was most simply expressed when considered independent of the after effect or drift. For molded bands originally 1 mm in thickness, 4 mm in width, and 10 cm in mean circumference, and with loads ranging from 30 to 836 grams producing elongations up to about 700 percent, the rates of expansion were found to lie between plus 0.0037 and minus 0.0550 cm per degree. The relation between the rate of thermal elongation, dl/dθ, and the stretching load, w, was found to be linear within the limits of the experiment, and the constants c and d in the empirical equation,have been evaluated, the former being negative in sign. The critical tensions for which there appeared no thermal change in length were 63 and 97 grams, respectively, for the particular shapes and compositions used. These tensions are equivalent to critical stresses of 7.72 and 11.9×105 dynes/cm2 (based on the cross section of the unstretched band) or of 11.2 and 17.2 lb./in.2, respectively, for the two compounds. Extrapolation by placing w =0 does not seem justifiable because when the resulting rate is divided by one‐half the circumference of the bands the magnitude of the coefficients of linear expansion turn out to be several times the generally accepted values based on volumetric determinations.
8(1937); http://dx.doi.org/10.1063/1.1710268View Description Hide Description
A rapidly moving recording galvanometer (Micromax) is used to maintain balance in a Wheatstone bridge. Received radio signals unbalance this bridge, so that an automatic record of voltage signal strength is taken. On daylight signals from nearby stations, the signals fluctuate rapidly during each day and night signals show a relationship dependent upon the barometric pressure. The latter observations agree with those made by Martyn in Australia. As autumn approached in 1936 every station recorded showed an increase of signal strength during the daylight hours.
8(1937); http://dx.doi.org/10.1063/1.1710270View Description Hide Description