Instruments \ Gravity \ 

Description:

This instrument was used to make gravity measurements in a submarine during 1935-1969.

How does it work?

The Vening Meinesz Pendulum Apparatus has three pendulums in a line. The line of the pendulums is oriented perpendicular to the main axis of the submarine to minimize the effects of the ship's motion. The two outside pendulums swing exactly opposite to each other so that the swinging of the pendulums will not transfer any significant energy to the Gimble Frame in which the apparatus hangs. Since the two pendulums are swinging in opposition, they cause acceleration in opposite directions and substantially cancel. The amplitudes of these pendulums are made as equal as possible. The pendulums were labelled 4 and 5 on the outside and 6 in the middle. The middle pendulum, 6, is left hanging motionless, initially, and picks up motion as the acceleration of the submarine affects the pendulums. The photographic trace from the pendulums is made up of the main pendulums of a reflection from the number 4 pendulum and the number 6 pendulum so that one is observing the difference in motion between the two pendulums. This makes what we call fictitious pendulum 4/6, and, to a first order of approximation, is a pendulum that is not disturbed by the horizontal accelerations in the direction of the plane of the swinging pendulums. Pendulums 5 and 6 are also recorded and is the lower trace on the record and it is made up of the fictitious pendulum 5/6 between pendulum 5 and pendulum 6. Since one has to swing pendulum 4 and 5 in phase opposition to cancel accelerations from the pendulum swings themselves, this is a redundancy of observations, however, it does provide additional statistics on the observed gravity and so it was used as an additional calculation. The interruption of the light beams is visible on the pendulum traces. The wide interruptions are tenth second marks and the small interruptions are hundredth second marks so that timing along the trace can be obtained to something on the order of a thousandth of a second.

The pendulum period, as we would call it today, is one second, however, for historical reasons they were called half-second pendulums because when pendulums were first used, the swing was timed from one extreme to the other, and called the period of the pendulum. (The period is now defined as the time from going to one extreme to the other and back to the first). So even though these pendulums take approximately one second for a round-trip swing, they were called half-second pendulums. When the nomenclature for the period was changed, the nomenclature for the pendulum was not.

Pendulum number 6 was allowed to remain as motionless as possible at the start of the record, thus picking up the motions of the Gimble Frame which was induced by lateral accelerations on the submarine. This motion is measured from the record which is minimal on the base station record where the submarine is not being accelerated appreciably, but it is measured to make a correction for amplitude that is needed in the computation of the gravity values from these pendulums.

The heavy lines (24A-D) that are superimposed on pendulums 4/6 and 5/6 are long period pendulums, the piece of apparatus that lies between the camera and the main pendulum box, which amount to extremely sensitive level bubbles. They are pendulums that have periods of 24 to 25 seconds and remain substantially level while the apparatus may swing a little bit in the wave period which tends to be in the period range of 4 to 6 seconds. These pendulums do not pick up any appreciable amplitude when the apparatus is displaced by wave motion and provide the information that is necessary for the second order correction which was first pointed out by B. C. Brown of Cambridge, England. Consequently it has been called the Brown Correction. The trace superimposed on pendulum 4/6 is the thwart ships instrument and the dark trace superimposed on the pendulum 5/6 is the fore and aft indicator. At the base station, of course, these picked up negligible amplitudes, but in a real sea record would show substantially, the residual wave motion that affects the submarine at its submerged depth.

Since the timing is inserted on the light trace, the pendulum record is also showing the time, simultaneously. The appearance of the diamond patterns indicated from the larger interruptions approximately 1-1/2 inches in size represents the number of pendulum swings in which a tenth of a second in time is lost or gained depending on whether the pendulum period is a little longer or a little shorter than one second from the timing of the crystal chronometer. It would be more accurate to represent the difference between the timing of the pendulum and the timing of the crystal chronometer. The technique of reading the record would be to read about six of such crossings at the beginning of the record and six at the ending of the record and where they cross the central line of the record. The diamond patterns, of course, indicate where the center line of the record is. By drawing lines in the middle of the diamond patterns, the central position of the pendulum swing is determined. When one is at sea these seemingly uniform patterns that we see on this record are disturbed by accelerations and so one has to draw the lines as the mean line of the oscillatory wave motion of these big interruptions. Records are made for a period of approximately 40 minutes. There are approximately 2400 swings in the total record, and since we feel we can measure timing from these time marks to about a thousandth of a second we are measuring the period of the pendulum to the order one part in two million so that the gravity value would be represented by approximately one part in one million.

How was it used? The Vening-Meinesz Pendulum Apparatus was used on submarines to measure gravity at sea. It was necessary to do this on submarines to get below the surface of the ocean where the wave disturbance was minimized. Normally, it was operated at about depths of 100' when waves were normal size, but observations as deep as 400' were necessary when the waves were more serious on the surface. In such cases the submarine still might roll 1 degree on either side of the vertical and still might do some pitching. Because of the motions of the submarine the apparatus is hung in the Gimbal Frame which is #18 (a second similar Gimbal Frame Disassembled is #17.) The Gimbal Frames were unclamped. When unclamped with the counter weights positioned properly would approximately level the apparatus in the Gimbals and then the rolling and pitching of the submarine, due to the operation of it and due to the wave motion would not appreciably effect the level of the apparatus. Normally when we operated at sea, we got stabilized at depths and then allowed the Gimbal apparatus to come to its level usually checking it with our hands at the bottom of the frame to let it settle down. As soon as it was settled down, we would then relevel the apparatus using the level screws on the bottom to do the final leveling in the Gimbal frame. Once it was level we would unlock the pendulums and lower them.

Pendulum operations were done on the submarines of the US Navy and in one case of the British Navy. Dr. Ewing made the first observations with this equipment in 1935 on the USS Baracuda. From 1946 to 1969, I, Dr. Worzel and my students made measurements on about 35 or 40 different submarines in different parts of the world so that we finally ended up with approximately 3000 observations throughout the oceans of the world.

After the pendulums were unlocked we lowered the pendulums onto the knife edges. This had to be done very carefully. We lowered the long period pendulums whilch are located on top of the main pendulums and below the camera. Thus all of the pendulums were supported on the knife edges. The long period pendulums were allowed to sit idle as they amount to a very fancy level bubble which measures any residual motion of the pendulum case relative to the vertical. The long period pendulum are set for period of the order of 25 seconds and the wave motions had periods of 4 to 12 seconds. During the period of a wave the pendulums do not move an appreciable amount. These are necessary to make what we call second order corrections because as Ben Bronwn of Cambridge, England, pointed out, there were square terms that affected the pendulums observations and these corrections amount to between one and a maximum of 20 milligals. In a normal operations, they were on the order of 3 to 4 milligals.

The first order terms were observed by the damped pendulums inside the main pendulum apparatus and were used to correct for the first order terms. Once we had all the pendulums hanging on their knife edges this includes the two damped pendulums inside the case, one for fore and aft motions and one for thwart ship motions and the two long period pendulums on top of the case again one for fore and aft ship motions and one for thwart ship motions as well as the main pendulums. We would cock the main pendulums by the controls on the front of the pendulum case. This had to be done very carefully because manipulating the controls would likely move the Gimbal Frame and so one had to develop a very careful touch to be able to turn these levers without disturbing the Gimbal Frame. When the pendulums were cocked and in position, we would then snap off the pendulums swinging back and forth approximately 1 cm total amplitude.

The bob is about 25 ems. from the knife edge.

These pendulums had about a second period while they were swinging. When pendulums are swinging there is a screen on the bottom of the camera case that one could see to check the motions. First of all the light was lit before all of the above and would reflect off all the mirrors of the various pendulums and appear on the screen at the bottom of the camera. Once the pendulums were started one would check on the screen that everything looked right for the operation; when it looked right the camera release mechanism was operated and that would put the spots on the photographic paper in the camera instead of on the viewing glass. Of course, the camera motor would have been started just before this. The light beam shone through a wheel of the chronometer which had ten holes in it. These interrupted the light with an extra long interruption once every ten holes to make it easy to count every tenth of a second. The chronometer had marks on it so that its rate could be checked to a thousandth of a second against WWV as a standard time signal. The light was interrupted there to all of the various pendulums on the record so that the timing was on every trace on the record. A record would normally be made for about 40 minutes duration which would be about 2400 swings of a pendulum and since we were able to time the pendulum to approximately a thousandth of a second we were able to obtain time to about l part in 2 million which allowed us to make the gravity measurements to about 1 part in a million.

When the observations were completed, the situation was reversed, all the pendulums were stopped using the same cocking levers used to start the pendulums. When the pendulums were stopped they were released very gently so that the pendulums came to their zero position and were motionless; then clamps were engaged clamping the pendulums. When the pendulums were clamped (they were quite rugged in that position), we then clamped the Gimbals and everything would be ready to surface the submarine. Usually we would tell the submariners that we were done with our observation as soon as we were completed and we had the instruments all clamped. We had removed the film from the camera by the time the submarine had reached the surface. To remove the film from the camera there was a set of 'minnies drawers' on the front that you could put a container inside, go inside with your hands, cut the film with a razor blade that was provided for that purpose, remove the film from the take up spool, put it in the container, return the take up spool to the camera and re-fix the film, closing the camera and then removing the container. The film was later developed in a developing box we usually had in the same compartment, but not right handy. The compartment was lighted so we had to have a set of minnies drawers in the developing kit also. The paper for recording was photographic paper which we obtained from the Kodak Company, and it was as I remember 12 cm wide and came in lengths of 200 feet. For a normal record we would make the record approximately 5 feet long.

The timing originally was provided by marine chronometers as Vening Meinesz built the apparatus. In 1935 Dr. Ewing. when he was going out on the Baracuda, persuaded the Bell Telephone Laboratories to build a crystal chronometer which was able to keep time to 1 part in a million using a quartz crystal. The crystal. chronometer that Dr. Ewing used is #10. Probably quartz clocks and watches available today were evolved from this crystal chronometer.