Try to place a plate topside up on a thin stick and keep it in a position of equilibrium. Nothing will come of your efforts. However, this is a favourite trick of Chinese jugglers. They succeed in performing it with several sticks simultaneously. A juggler doesn’t even attempt to maintain his thin sticks in a vertical position. It appears to be a miracle that the plates slightly supported by the ends of the horizontally inclined sticks do not fall and practically hang in the air.

If you have the opportunity of observing jugglers at work at close range, note the following significant detail: the juggler twists the plates in such a fashion that they rotate rapidly in their planes.

Juggling maces, rings or hats, the performer will in all cases impart a spin to them. Only then will the objects return to his hand in the same state in which they were put at the beginning.

What is the cause of such stability? It is related to the law of conservation of angular momentum. For when there is change in the direction of the axis of rotation, the direction of the angular momentum vector also changes. Just as a force is needed to change the direction of velocity, so a torque is needed to change the direction of rotation; the faster the body rotates, the greater the torque required.

The tendency of a rapidly rotating body to preserve the direction of its axis of rotation can be observed in many cases similar to those mentioned. Thus, a spinning top does not tip over even if its axis is inclined.

Try to overturn a spinning top with your hand. It proves to be not so easy to do this.

The stability of a rotating body is utilized in the artillery. You have probably heard that gun barrels are rifled. An outgoing projectile spins about its axis and, because of this, does not “tumble” through the air. A rifled gun gives incomparably better aiming and greater range than an unrifled one.

It is necessary for a pilot or a sea navigator to always be aware of the location of the true terrestrial vertical relative to the position of the airplane or the ship at the given instant. The use of a plumb-line is unsuitable for this purpose, since it is deflected during an accelerated motion. A rapidly spinning top of special construction is therefore employed-it is called a gyrovertical. If we set its axis of rotation along a terrestrial vertical, it will then remain in this position, regardless of how the airplane changes its position in space.

But what does the top stand on? If it is located on a support which is turning together with the airplane, how can its axis of rotation preserve its direction?

An apparatus like the Cardan suspension Figure 1 serves as the support. In this apparatus, with a minimum of friction at the pivots, a top can behave as though it were suspended in air.

With the aid of spinning tops, it is possible to automatically keep torpedoes and airplanes on a given course. This is done by means of mechanisms “watching” the deviation of the direction of the torpedo’s axis from that of the top’s axis.

Such an important instrument as the gyrocompass is based on the application of the spinning top. It can be proved that under the action of the Coriolis force and friction, the top’s axis eventually settles down parallel to the Earth’s axis, and so points to the North.

Gyrocompasses are widely applied in navigation. Their main part is an engine with a heavy flywheel which does up to 25000 rpm.

In spite of a number of difficulties involved in the elimination of various hindrances, in particular those due to the pitching of a ship, gyrocompasses have an advantage over magnetic compasses. The drawback of the latter is the distortion of the readings because of the influence of iron objects and electrical appliances aboard the ship.