University of Cambridge Museums

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How Strong is a Piece of String?

One of my first tasks when I started my job as Museum Technician at the Whipple Museum of the History of Science last summer was to launch into research and become a mini expert. Not in telescopes, planetariums or atomic models (that was all still to come)… but in rope.

One of our objects at the Whipple Museum needed a new rope to function. It’s a beautiful telescope from c.1790, and you can adjust it in its stand by means of a pulley system. Unfortunately no cord lasts forever and the old one broke, leaving the poor telescope in one position. That just wouldn’t do.

It’s not as easy as popping down the shops for some more rope. This is a historical object in a museum collection, so it’s our job to have a good think about what we should do next. The old cord was a twisted hemp twine of about 2-3 mm and it could be anywhere between 70 and 200 years old.

Questions I was asking myself:

Should the new rope be like the old one: a natural fibre twine? That would look authentic and be what the original manufacturer intended to be used, but how long would it last? How often would we need to replace it so it doesn’t break again? Is it okay to replace the twine with something a visitor could think was original, or is that a little deceptive?

Should we use a synthetic rope instead? It would have greater tensile strength, so we’d worry less about the cord breaking. What kind of synthetic fibre should we use? Polyester, nylon, polypropylene, aramids, acrylics and polyethylenes all have different properties. Some elongate under constant load and others are slippery. Some can only be obtained in bright colours, which automatically disqualifies them (not everyone likes hot pink with mahogany and brass!). Another worry is ageing properties: how long would a synthetic rope last with organic acids being released daily from the wood around it? Nobody has really tested that and the quality of any given synthetic rope varies with manufacturer, fibre type, and batch.

Would a steel wire be better? You wouldn’t worry about that breaking and it could handle considerable loads without a problem. With the right coating it wouldn’t even react with the brass pulleys or with the acids from the wood, which would make it considerably more conservation grade. It wouldn’t look original but it would last. Can we find a steel wire that’s flexible enough to fit into the existing pulley system though?

How strong is a piece of string? How strong does it need to be?

herschel_03_string_optionsSo I spent a lot of time calculating tensile strengths, discovering the many ways in which to twist or braid rope and what that does to its strength and flexibility, reading up about rope stretch over time, and talking to sailors and other rope experts. I consulted a few engineers as well, who likely thought museum people have the oddest questions.

With the help of Des Pawson MBE from the Museum of Knots and Sailors’ Ropework in Ipswich and a manufacturer of steel wire happy to send us samples I shortlisted a few alternatives.

After mulling it over in museum staff meetings we settled on the long-term option: steel wire. We found some very flexible 2 mm wire rope that happily bent enough to work in a relatively tiny pulley system, and it is many times stronger and more durable than any of its alternatives. Admittedly steel wire isn’t as easy to work with as it sounds: you need special tools and stoppers to tie it off, as you can’t just knot it like you can do with twine.

herschel_04_being_repairedThe big challenge was to actually replace the old cord. After carefully unwinding what was left I calculated there had originally been 12 metres of twine on the spool and through the pulley mechanisms. Steel wire behaves very differently from rope so it took a lot of effort and endless untangling from me and my colleagues as we started winding the wire onto the spool and gradually threaded it onto the pulleys. It took three people a couple of hours of solid work before we could secure the wire, stand back, and put it to the test. It worked!

I’m actually very pleased with how it looks as well. I thought it would be a little too shiny or that it would stand out against the mahogany and brass, but in fact the wire blends in quite nicely at a few paces away. That means we’ve successfully stuck to the age old conservation principle: it should look natural from afar but up close you should be able to tell that something has been repaired.

The original cord is safely tucked away for anyone who wants to examine it in the future and the Herschel telescope now stands proud in our Main Gallery again, looking to the stars (well technically it gazes towards our Globes gallery).

Come and see for yourself!

P.S And the answer to the question is that a piece of steel wire string is very strong indeed: it’ll take over 200 kg to break it, but it’ll safely hold 35 kg for a very, very long time. Luckily the telescope only weighs 7kg without its mirrors!

Jenny Mathiasson, Museum Technician, Whipple Museum of the History of Science

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This entry was posted on March 2, 2016 by in Behind the Scenes, Culture, Goal 1, News and tagged , , .
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