Regulator is a precision clock that watchmakers used in the past to observe the operation of their watches. Their precision can reach the level of quartz precision.

There are two main reasons for this high precision. One is that these clocks remain fixed and thus undergo no external shock. The other is the large size of their resonator which is therefore in this type of clock a pendulum. The regulators have the particularity to beat the second. Thus the watchmaker will be able to hear the second and transpose it to the time he will read on the observed timepiece.

To beat the second the theoretical length of the pendulum between its attachment point and its center of gravity must be exactly one meter. In practice this distance will be slightly less to compensate the delay created by the escapement. The frequency of these clocks is 0,5 Hz, because they make an oscillation in two seconds what corresponds to 3’600 vibrations/hour thus one vibration per second. These clocks also have a very low amplitude of about 1.5 degrees which is not a problem because they do not suffer any external shock and therefore a long pendulum is preferred to a high amplitude.

Another reason for the high accuracy of these clocks is that they often have a mercury temperature compensation. Thus the lens of their pendulum often looks more like a container, not necessarily translucent, which can contain about half a liter of mercury. This volume of mercury represents a mass of about 7 kg. This mercury will make the thermal compensation. Thus when the regulator cools down the pendulum will shorten but the mercury will go down. Thus the distance between the point of attachment of the pendulum and its center of gravity remains the same. On the contrary, when the regulator warms up the pendulum will lengthen but the mercury will rise. Thus the distance between the point of attachment and the center of gravity of the pendulum remains the same again.

There are also often fine adjustment mechanisms that crown the precision of these regulators to the extreme. Their dials are also known for a specificity, they often have in the center only the minute hand, the hour and the second being often on two other off-center dials. This type of display has also appeared since the 20th century on wristwatches.