As early as the 16th century, renowned astronomers and natural scientists were already studying the peculiarities and behaviour of the pendulum, or what was then called the gravity pendulum. In elaborate experiments and calculations, they came to the conclusion that the period of oscillation of a pendulum with thread suspension did not depend on the mass or shape of the pendulum body, but only on the length of the pendulum itself. A pendulum, once set in motion, always needs the same time for one oscillation. So it was obvious to use this effect in wheel clocks. At that time, clocks were relatively inaccurate and the search for the most precise oscillation system, by which time could be measured or displayed, was not finished.

Today we know various oscillation systems like the balance (for mechanical watches), the quartz (for watches with battery operation) or even the pendulum. The latter is still the most accurate mechanical oscillator which is still used for high-precision, fixed or suspended clocks.

In 1585, as mentioned above, Galileo Galilei made the discovery that the frequency at which a pendulum swings is determined primarily by its length and the local gravity surrounding it. In addition, he observed that the amplitude of a pendulum has no influence on the duration of a pendulum oscillation; this is described today with the term isochronism. With regard to the isochronism, however, we know today that it is only approximately valid for very small pendulum oscillations.

Because a pendulum, once set in motion, permanently loses its amplitude due to various disturbing influences such as gravity, air resistance or even the friction of the pendulum suspension, force must be applied to it at regular intervals. In the case of a fine pendulum clock, a weight is responsible for this, which transmits the force to the pendulum by means of the clockwork. At this point it should be mentioned that the weight in this case is the absolute energy store to be favoured, as it has a permanently constant (weight) force.

If a pendulum is now installed in a fixed clock, is driven with constant force and its length is adjusted to the exact geographical position, the clock should display the time with maximum accuracy. However, this is not automatically the case! Now further disturbing factors come up to the existing system. First and foremost these are temperature fluctuations, air pressure fluctuations and also fluctuations in humidity.

The disturbing air humidity can be remedied very quickly by painting wooden pendulum rods or even using metal rods.

A bigger problem are the deviations due to temperature fluctuations. At high temperatures materials expand, the pendulums become longer and thus slower, the clocks slows down. At cold temperatures the effect is reversed. The first step was to use special types of wood that did not react very well to these fluctuations. This was followed by experiments with complicated and costly constructions of various rods made of different metals that were supposed to compensate or equalize each other in order to avoid the change in length of the pendulum rod due to the temperature, the so-called grid-iron pendulum. Finally, at the end of the 19th century, the French scientist Charles-Edouard Guillaume discovered an iron-nickel alloy that was groundbreaking.

The coefficient of thermal expansion of this new material was 10 times lower than steel and 5 times lower than selected woods. This new “invariable” metal was therefore, obviously, given the name Invar.

Since the invar nevertheless has a low thermal expansion, clocks with rate deviations of several seconds per month were possible, but better values could not be achieved by invar alone.