This device is made from a set of four Invar wires, two weights, two cross pieces and legs. It is a precision measuring instrument developed at the end of the 19th century in response to increasing requirements for the accuracy of long distance measurements.
Invar is an ally of iron and nickel that was discovered in the 19th century. It has a low thermal expansion coefficient.
The device shown here dates from the 1950s.
A wire is 24 metres long and therefore, under ideal conditions, the distance measured will be a multiple of 24 metres.
The wire is held taut by two 10 kg weights so that its length remains constant.
In order to install the device, there needs to be a support at both ends.
For the measurement, the wire is compared to the distance indicated on the ground with aligned bearings.
The geodetic bases for the triangulation may range from several hundred metres to a few kilometres in length.
The results are recorded multiple times on the vernier scales at the extremities of the four calibrated wires of the device.
The different values are then processed statistically.
In calculating the distance, factors such as the temperature, wind force and speed, the elevation, and the horizontal grading and alignment of the bearings are taken into account.
This device is the last example of an instrument that is used to physically compare against the distance to obtain a measurement.
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