An apparatus for studying the damping of low frequency torsional oscillations

FOR studying low temperature polymorphic transitions which occur, for example in lithium and sodium, 1,2 it is intended to use the determination of the changes in decrement of torsional oscillations, as is done for a number of metals with relatively high transition temperatures.3. 4 Since one can count on success in such measurements only at very low frequencies, and the specimens cannot be prepared sufficiently thin and long, it was necessary to turn to the method of a connected pendulum, the frequency of which depends very little on the shape, dimensions, elastic, and plastic properties of the specimen. In a previous work 5 the connected pendulum method was applied for studying flexural oscillations. The present work is concerned with the application of this method to torsional oscillations. Apparatus for studying torsional oscillations at low temperatures are described by several authors. 6-s In all these the frequency of oscillation depends mainly on the elastic properties of the specimens. If one is not concerned with determining the elastic moduli, it is possible to construct an apparatus such that the frequencies of oscillation are determined in advance by the components of the apparatus. The system will then correspond to the connected pendulum principle s described above. In the apparatus described the oscillations are recorded electronically, using a capacity probe. The apparatus is shown schematically in Figure 1. The cylindrical specimen 1 (/work = 30 mm, diameter 3 mm) with broadened ends (10 mm diameter) is held in clamps. The lower clamp 2 is mounted at the bottom of a detachable metal case 3. The upper end of the specimen is connected to the rocker 6 by the clamp through the hardened steel rod 4 and the membrane 5. The rocker is hung on a similar rod 7 fixed to the lid of a metal case 8. Membrarie 5 is used to limit the axial load on the specimen during assembly and during temperature changes. Free damped oscillations are excited by electromagnets (one of which is shown in Figure 1) which provide a turning moment when their circuits are switched on for a short time, and disturb the system