Invar Pendulum

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Item Invar Pendulum
Item code
Description On Display Board
Donor Continental Oil
Location Houston Geoscience Center
Serial number
Taxonomy GSH277

Pendulum Field Apparatus.--The following is a brief description­ of the pendulum apparatus as developed by the Gulf Oil Corporation Each unit consists of two pendulums in a single case. The periods of the two pendulums are carefully adjusted so that throughout a normal run of up to 1 hr. the total number of swings will be within a small fraction of one swing of the same. The pendulums are started swinging in opposite phases, and because of the equality of the periods this phase relation is maintained throughout the run. The primary purpose of this arrangement is to eliminate sway of the case which would result from the reaction a single pendulum on the case. With a double pendulum the forces tending to cause sway from the two pendulums are equal and opposite and compensate each other. Amplitude corrections are eliminated by always starting the pendulums with the same amplitude. The decrement is quite constant so that the final amplitude also is the same from one run to another. Electrostatic disturbances are eliminated by use of a small amount of radioactive material in the case which renders the residual atmosphere conducting and dissipates any electrical charge on the pendulums. Also, it is the practice in field opera­tions to use two complete sets of apparatus so that at each station two independent values of gravity are determined. . The pendulum itself consists of a fused quartz bar which is care­fully designed as a "minimum pendulum”. The bar is carried by a quartz knife-edge resting on a solid Pyrex glass flat. The pendulum period (complete cycle) is about 0.9 sec. The motion of the pendulum is recorded by a beam of light reflected from a mirror surface polished on the upper end of the quartz bar. The pendulums are mounted in a vacuum-tight cast-aluminum case. Clamping, starting, and stopping of the pendulums are all done from the outside through vacuum-tight stuffing boxes with­out disturbing the vacuum. The cases are tight enough so that a vacuum of 0.1 mm. of Hg can be maintained for months with­out pumping out. The pendulum case is surrounded by a net of electric heating wire and is covered on the outside with a thick eiderdown blanket. This permits maintaining the case at a constant temperature, the heating power being supplied by auto­mobile storage batteries and controlled by a thermostat mounted on the metal case. The temperature is kept constant to about 0.1 degrees C. A "minimum pendulum" is proportioned so that its period is a mini­mum. For a physical pendulum, oscillating about a knife-edge at a distance from the center of gravity, the reduced length, or length of the equivalent simple pendulum.