As you know the SI unit of force is the
newton (N), and is defined as the force required to accelerate a mass of 1 kg at a rate of 1
ms-2.
In practice it is not convenient to create an
acceleration of one metre per second per second on an object having a mass
of one kilogram in order to produce the standard of force of one newton.
Instead, to produce a calibration for the unit of force, laboratories use
known masses (deadweights) which will produce a known force when subjected
to the effect of the local acceleration due to gravity. Machines which
generate force in this way are called deadweight machines. The forces are
measured using transducers.
In the UK the National Physical laboratory (NPL)
has a number of force machines providing UK standards in the range from 1.5 N to 30 MN.
The forces from 1.5 N to 1.2 MN are produced using deadweights and the higher forces are
generated by using hydraulic machines. With these machines, NPL provides a calibration
service to industry for force transducers and strain cylinders.
These machines are
also calibrated themselves and compared with similar machines in other
countries.