I am not able to understand what is pendulous suspension and how centre of gravity and centre of suspension are related in magnets and their errors. Can someone explain this in layman language or could share the link which gives good 3d animation of the same.

  • $\begingroup$ Research the "Bohli" compass and understand why it is free of turning errors-- $\endgroup$ Commented Jan 22, 2021 at 14:07

1 Answer 1


This video covers it pretty simply. The compass magnets are attached to the bottom of an upside down cone, which is resting on a spike. The weight of the compass magnets, and the center of gravity of the assembly as a result, is near the bottom of the cone and the "hang point" is up near the top at the tip of the spike.

It's a cone pendulum; basically a lady in a hoop skirt, out on the dance floor (put a couple of bar magnets alongside at at the bottom of the skirt, stick her in a vat of kerosene to dampen the skirt's movements, and there's your compass-lady).

The pendulum arrangement keeps the compass magnets more or less level to the earth even if the spike it's on is tilted (mostly to accommodate changes in pitch attitude with speed and stabilized climbs and descents), within the range of movement of the cone. Compass units in ships (and very early aircraft compasses) achieve this by having the compass float on water (or some other fluid) within the housing so the compass can stay horizontal while the ship pitches and rolls.

The various turning errors relate to how the pendulum reacts to accelerations and apparent gravity (think of the lady's hoop skirt tilting off vertical as she starts to move, for example), the C of G of the cone being below the hang point, while at the same time having the magnetic alignment forces acting on the magnets. The result is readings that wander in specific directions (acceleration/deceleration and turning errors) if there are any lateral accelerations going on, and while the magnets are rotating, because of the "struggle" going on between the magnetic forces and the inertial forces. The compass reading is only valid when the inertial forces have gone away, when there is a steady state condition of no turns and constant speed.

  • $\begingroup$ Thank You John ! $\endgroup$ Commented Jan 23, 2021 at 8:04
  • $\begingroup$ John the video was very nice and your explanation was superb. But in the video they mention that in compass digits are in reverse order I am not able to understand that. $\endgroup$ Commented Jan 23, 2021 at 9:09
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    $\begingroup$ It's because you are looking at the lady from behind outside the skirt, and she isn't turning but just stays aligned with north, while you are moving around her from the outside. The N is printed at the BACK of her skirt and you see it when south of her. If you move around so you're facing west, you're moving around to the EAST side of her, and that's where the W on her skirt is, to tell you you're facing west. Put numbers between N and W and it counts down from 360 at N to 270 at W. You are turning to the left, but the progression of the numbers IN YOUR VIEW is to the right as a result. $\endgroup$
    – John K
    Commented Jan 23, 2021 at 14:04
  • $\begingroup$ Hi John. You have explained and shared me good content. I am facing a lot of difficulty in understanding gyros and gimbals and their working, axis of freedom etc. Do you you have a video link for that... $\endgroup$ Commented Jan 25, 2021 at 17:53
  • $\begingroup$ Just go on youtube and search "aircraft gyro" and it'll present a number of videos. $\endgroup$
    – John K
    Commented Jan 25, 2021 at 19:28

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