Volume 9, Issue 11, 01 November 1938
Index of content:
9(1938); http://dx.doi.org/10.1063/1.1710379View Description Hide Description
In a commercial method of starting the Cooper Hewitt Hg vapor lamp, a current of about 0.7 amp. is first made to flow in a circuit containing an inductance and a Hg vacuum switch or ``shifter.'' This shifter is then opened magnetically and the resulting arc therein is unstable and begins to go out. But the inductance acts to keep the current flowing and develops a high voltage which is impressed as a negative kick of about 1000 volts, in the average case of starting, on the Hg pool cathode of the main tube. This kick, as a result of the action of a strip of tinfoil on the outside of the cathode bulb surrounding the Hg pool edge and connected to the main anode, starts the arc. The whole process results in the transfer of nearly the entire current from the shifter arc to the main arc in a time of the order of a few microseconds. The initiation of the cathode spot is attributed to the action of a high electric field at the Hg pool edge existing between this edge and the tinfoil on the outside of the glass. In accord with this theory is the fact that anything which can be done to cause the Hg to wet the glass at the pool edge at a number of points (thereby increasing the field) such as by baking a little carborundum powder on the glass at the edge, helps starting enormously.
If the anode of the tube is disconnected, simple electroscopes show that the walls of the tube everywhere become charged to a negative potential of the order of 10,000 volts as a result of the kicks. High speed electrons are thus evidently shot up the tube. Ionization produced by these probably forms the beginning of the positive column. After initiation, the current in the tube rises to a maximum at a rate limited by the inductance in the circuit and then decreases to a low value sometimes going out altogether. This decrease is shown to be due to an extreme deficiency of Hg vapor in the tube caused by the electrical clean‐up of Hg on the walls. Violent voltage surges set in across the tube at this time and a hissing noise is heard. The region of aggravated low pressure can be seen as glowing weak and reddish compared to the blue white of the rest of the tube. This appearance vanishes when, as a result of the rising vapor pressure due to heating, the hiss and surges stop. The surge period may endure many seconds in cold weather before the arc is able to pick up.
9(1938); http://dx.doi.org/10.1063/1.1710380View Description Hide Description
9(1938); http://dx.doi.org/10.1063/1.1710381View Description Hide Description
I. Theoretical considerations.—The conditions which determine whether a homogeneous liquid will solidify into the crystalline or into the vitreous state are discussed and a brief account of the rôle of the speed of crystallization,V, and that of grain formation, N, is given. These concepts are applied to the causes of the death due to crystallization of nondehydrated organic cells, and the physical conditions of cooling are developed under which such organisms can be solidified with a minimum of crystallization, assuring to this extent the preservation of their potential life.
II. Technical considerations.—Methods for obtaining cooling rates up to 104 deg. sec.−1 applicable to organic cells in a highly dispersed state are developed. As subject for the experiments, pure strains of yeast (Saccharomyces cerevisiae) were chosen on account of the high sensitivity to low temperatures. As death criterion, the permeability of the cells to a standard aqueous solution of methylene blue was chosen, the death rate was thus determined microphotographically or with a counting chamber. Different techniques of exposure favoring and preventing crystallization are developed.
III. Experimental results.—The rate of cooling of suspensions of living cells in aqueous media was varied from 1 to 104 deg. sec.−1 and thus conditions favoring either crystallization or vitrification of the cellplasma were produced. The death rate at an exposure of −185°C was thus varied from approximately 75 percent to approximately 3 percent. The duration of exposure at this temperature was varied from 5′ to 6000′; no appreciable influence of the time of exposure was found. The temperature to which the cells were cooled was varied from −50°C to −252°C and it was found that between −185°C and −252°C the temperature did not affect the death rate. Above app. −150°C an increase of the death rate was found, and a dependence upon the time of exposure, since the speed of crystallization begins to be appreciable. Repeated exposures of the same cells under conditions not especially favoring vitrification were studied and the integral death rate, δ n (death rate after nth exposure) was investigated with respect to a constancy of the death rate (δ) at each freezing. It appears that the death rate for repeated freezings is not controlled by simple probability relations, but that other effects enter, such as weakening of cells by previous exposures and the selection of cold‐resistant cells by survival.
9(1938); http://dx.doi.org/10.1063/1.1710382View Description Hide Description
New synthetic dielectric materials, both of the ceramic and resinous type, exhibit, among other features, a high grade of homogeneity. This will not only affect the mechanical properties, the electrical breakdown strength and the dielectric losses due to layer mechanism favorably, but will reduce also losses due to dipole rotation. In case, namely, dipolar constituents rotate at all in the solid state, dissipation of energy will be caused by the solid friction, which has to be overcome. This latter, on the other hand, can be shown to be related directly to the imperfections of the material, thus indicating the beneficial effect on the dielectric loss angle of a homogeneous structure.
9(1938); http://dx.doi.org/10.1063/1.1710383View Description Hide Description