As discussed in gyro compass, when the spin axis is horizontal the CoG of the weight passes through the center of the rotor producing no torque but the spin axis does not remain horizontal. The earth’s rotation tilts the spin axis. When gyro axis tilts the CoG of the weight does not act through the center of the rotor and this weight produces a torque about the horizontal axis. This torque tends to take the spin axis towards the meridian. Adding a weight to the top of the rotor casing produces a number of undesirable effects. These effects become pronounced when a ship is subjected to severe movement in heavy weather. To counteract unwanted effects, an ‘apparent’ top weighting of the compass is achieved by the use of a mercury fluid ballistic contained in two reservoirs or ballistic pots.
As shown in above figure, each ballistic pot, partly filled with mercury, is mounted at the north and south sides of the rotor on the spin axis. A small-bore tube connects the bases of each pot together providing a restricted path for the liquid to flow from one container to the other. The ballistic system is mounted in such a way that, when the gyro tilts, the fluid will also tilt and cause a displacement of mercury. This action produces a torque about the horizontal axis with a resulting precession in azimuth.