"The needle to the pole" is a very picturesque expression to describe dependability or reliability. This describes a person who will always do what is right, whose influence will always be in the right direction. But if we are talking about the magnetic compass needle and the magnetic pole, our example falls short of being perfect. The orientation of a magnetic needle in the earth's magnetic field is not always unerringly accurate. Not only is the magnetic pole not the same as true north, but it is constantly shifting, so the pole today is not where it was 100 years ago. Also, the direction of the magnetic pole varies according to geographic location and external influences. So the accuracy of the needle to the pole is somewhat in question. Just why the magnetic poles should wander around is a matter of speculation. It is generally felt that the earth's magnetic field is caused by the effect of the earth's rotation on the central liquid core composed mainly of iron and nickel. The north magnetic pole seems to be drifting in a spiral toward true north, which is the direction in which the earth's axis of rotation is pointing.
Jean Bernard Leon Foucault (1819-1868), the French physicist, invented what is called thE "Foucault Pendulum." Such a pendulum, when set swinging, will continue to swing in the same direction in space regardless of the earth's rotation. At the north pole, its arc will appear to make a complete rotation in 24 hours, as the earth rotates beneath it; at the equator the arc appears to always swing in the same direction, since the earth is not rotating beneath it. In other words, it has an apparent infinite rotation time. The apparent rotation time of the arc for every geographic location between these two extremes will vary inversely as the latitude. For example, in San Francisco whose latitude is about 37 degrees north, the apparent rotation time of the pendulum arc is around 37 hours, and as one travels south, this time increases until at the equator it reaches infinity.
The discovery of this unique property of the pendulum resulted in the invention of the gyroscopic compass. Like the pendulum, the rotational axis of a gyroscope will always point in the same direction in space regardless of the earth's rotation. Once it is pointed toward true north, it will not deviate from that orientation, and it is not influenced by external factors.
Gyroscopic compasses have replaced magnetic compasses for all modern navigational instruments because of their accuracy and reliability. Thus, if we wish to speak of a person's reliability, we should modernize our conception to that of the gyroscopic compass. Then, a person fitting such a description would indeed be as true to duty "as the needle to the pole."
