Volume 3, Issue 1, 01 July 1932
Index of content:
3(1932); http://dx.doi.org/10.1063/1.1745076View Description Hide Description
The underlying causes of the losses occurring in dielectrics when subjected to alternating stress have long been a subject for speculation. There have been a number of proposals of methods for linking such losses with the dielectric behavior under continuous or d‐c. electrical stress. To investigate the validity of the suggestions for correlating the d‐c. properties of dielectrics with their a‐c. losses, the continuous currents flowing must be measured as soon as possible after the d‐c. voltage is applied to or removed from the specimen. The development of the amplifier‐oscillograph permits a much improved and satisfactory technique. With the apparatus and method described here, observations could be begun within one or two thousandths of a second after the specimen had had the d‐c. voltage applied, or had been short‐circuited and they could be continued as long as desired. By this method, a connected series of observations was obtained with a single cycle of voltage application, removal, and short circuiting of the specimen, greatly expediting the work and eliminating inaccuracies arising from changes in the dielectric sample during the long periods of time heretofore required for such studies. The apparatus employed consisted principally of a quick acting, spring‐actuated switch for operating the specimen circuits rapidly, and a sensitive‐amplifier‐oscillograph for recording the dielectriccurrents. On operating, the quick acting switch shunted the geometric charge of the specimen to ground for a brief interval after applying voltage to the specimen, or short‐circuiting it, and then connected the amplifier‐oscillograph to the measuring circuit. The observations could be continued as long as desired with a D'Arsonval galvanometer of high sensitivity. The amplifier‐oscillograph attained a sensitivity of 1.6×10−8 ampere per mm deflection, which is about 300,000 times that of the oscillograph alone. The vacuum tubeamplifier which accomplished this extension of range was in three stages, pure resistance coupled for the accurate reproduction of continuous currents. Many precautions were taken and tests made to ensure the reliability of this instrument and its connecting circuit apparatus.
3(1932); http://dx.doi.org/10.1063/1.1745074View Description Hide Description
When commercial vacuum tubes are used to amplify small low‐frequency voltages it is found that random disturbances of the order of 100 to 1000 microvolts are present in the anode circuit. These disturbances exist almost entirely in the range below 100 cycles per second, and therefore fix the minimum voltage which can be measured over this low frequency band from 10 to 100 microvolts. These disturbances are shown to be caused by any or all of the following: (1) insulating material in or near electron path; (2) irregularity of filament emission; (3) gas; (4) positive ions emitted by filament; (5) insulating or foreign deposits on grid wires. A tube has been developed in which the above effects are removed or reduced to a point where the disturbances are nearly that of the shot effect of the electrons, as limited by space charge. This allows the amplification of low‐frequency voltages of less than 1 microvolt over the entire frequency band below 100 cycles.
3(1932); http://dx.doi.org/10.1063/1.1745075View Description Hide Description
A vertical magnetometer is described which is believed to fill the need for an intermediate type between the dip needle and the Schmidt balance. It does not require a tripod and levels itself. The compass for orienting remains attached to the instrument; it may also readily be compensated for temperature. It utilizes a magnetic system suspended in jewels in or near its center of gravity; the magnetic vertical intensity is balanced by the torsion of a helical spring. The magnetic system is adjusted to the normal vertical intensity in the area under survey by a suitable setting of the spring and the anomaly is roughly proportional to the angle of deflection from the horizontal.
3(1932); http://dx.doi.org/10.1063/1.1745077View Description Hide Description
3(1932); http://dx.doi.org/10.1063/1.1745078View Description Hide Description
The presence of an extra‐heavy (or light) body in the sub‐surface warps the level surfaces up (down) over the body and the lines of the vertical toward or into (away from) itself. The Eötvös torsion balance consists of a calibrated platinum‐iridium torsion wire which supports a beam of negligible weight; a weight is attached to one end of the beam and a second weight is suspended from the other end. Curvature of the level surfaces and curvature of the vertical each produce at the two weights horizontal components of gravity which are respectively equal and opposite in actual but the same in rotational direction. The torque produced by the curvature of the vertical is a function of the difference of the radii of curvature of the level surface in the directions of major and minor curvature. There has been only one slight important improvement of the torsion balance in the past twenty‐five years. There is room for improvement in the avoidance of temperature effects and in speeding up in the time necessary for observation. The mathematics of the torsion balance and of the gravitational effects of simple geometrical bodies is simple and developed. The main field of research with the torsion balance lies mainly in the geological‐geophysical field of interpretation, although there are still unsolved problems in the mathematics of interpretation.