As a BBC Horizon programme recently showed, the question "How long is a piece of string?" is more complicated than it first appears.

Part of the filming of a recent episode of the BBC's Horizon took place at NPL, where the two presenters, Marcus du Sautoy and Alan Davies, attempted measuring a piece of string using a laser tracker.

Firstly, as with any measurement problem, the exact definition of what is to be measured has to be decided. In the case of a piece of string, this requires consideration of many aspects, such as:

• Is the length measured along the central axis of the string, or along one edge? The results will be different if the string is not straight.
• Where exactly are the ends of the string? This is especially important to decide if the string is frayed.
• Should the string be lying flat or hanging up? The way the string is supported can affect its length by stretching it.
• How straight does the string need to be during measurement? The length along the chord is different to that measured end to end along a straight line.

Next, relevant measurement standards, measuring equipment and their traceability need to be considered. Fortunately, NPL, has many standards and measuring instruments available. In filming the programme, the presenters looked at a range of length standards, including an Egyptian cubit, the UK copy of the prototype metre, length bars and frequency-stabilised lasers.

In the end, the film crew decided to use a laser tracker to perform the measurement, as this was one of the easier measuring systems to film and explain to the audience. Alan Davies moved the instrument's spherically mounted retro-reflector (SMR) to each end of the string in turn. At both positions, the laser tracker measured the 3D position of the SMR. A few button presses later and the distance between the two SMR locations was calculated and read out. This was the length of the string.

Whilst laser trackers are rarely used to measure the length of string, they are fast becoming a very useful measuring tool, used in a variety of situations. NPL is currently using laser trackers in a variety of situations and is setting up a laser tracker calibration and verification service.

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