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Clock watching

Neil Downie and Steven Chapman look at the complex nature of an everyday object and how it can be used to teach time and measurement

Tissue is not a simple substance. Look under a microscope and a jungle assaults the eye. There are fibres of different sizes - some tubes, some solid - at different angles, twisted and bent around each other. Out of this complexity, however, we can find a very simple effect at its heart.

Surprisingly, we can use this complex substance to measure time.

Activity 1: the tissue-clock element Basic equipment:

* paper-handkerchief tissues

* paper

* sticky tape

* water

* non-permanent marker

* Petri dish

The tissue-clock element is a piece of four-ply paper-handkerchief tissue, 20cm long, trapped by two strips of sticky tape.

To form the element, stretch a 30cm length of sticky tape, sticky side up.

Cut a strip from the edge of a two-ply tissue and fold it lengthways, so you have a double-thickness piece about 20cm long. Draw a pattern of dots along the tissue using a water-soluble felt-tip pen.

When dry, lay the tissue lengthways on top of the sticky tape so only one end juts out. Cut a second piece of tape, identical to the first and lay it over the tissue. Make sure there aren't too many wrinkles, and that the tissue is well sealed in the tape along the sides, except for the tip.

Students can now try out their clocks. To set the devices going, mount the strip on a piece of paper horizontally, with the tip of the tissue in a Petri dish of water, and watch. You may find it surprising that the water travels so far along the tissue, given time.

Get students to record on the mounting paper the distances travelled after equal time intervals (say one minute). The ink from the dots shows the progress of the "wet edge". Students can record on a graph wet-edge position against time. I found that it took about 10 minutes for water to get along a 23cm strip of tissue.

* Ask students why the water does not progress at a steady speed. Does the water travel the first 10cm in the same time as it does the second 10cm?

* Ask students to try measuring how consistent the tissue clock system is.

Even with clock elements as identical as possible, do you get identical travel times? What factors will affect the time? Length is important but does width make any difference? What about the thickness of the tissue?

Activity 2: cooler tissue clocks

Try making a tissue clock using iced water. Temperature must affect the result, because it affects the surface tension and viscosity of water - but by how much? Get the students to predict what might happen.

Older students can look up the variation of water viscosity and surface tension with temperature in a data book, such as Tables of Physical and Chemical Constants by George Kaye and TH Laby (see box, right). Surface tension goes down with increasing temperature, as does viscosity. But the tension doesn't vary as much as the viscosity.

Activity 3: rising-damp clocks

Ask some students to try the element in the vertical direction. Is it the same? Is it, as one might expect, quicker downwards than upwards? Is there a difference between up and down? Ask students about rising damp in buildings.

The science and the maths

The speed at which the water travels in tissue fibres is not uniform, but it is averaged over hundreds of different fibres. We can understand the process roughly by assuming there is a capillary pressure driving the water, and that the water is flowing in approximately tube-like channels in the tissue. The pressure must act on an increasing length of pipe, which has the effect of slowing the fluid flow as the water travels further.

Suppose the wet edge exerts a capillary pressure P on the column of liquid, length L. Ask students where that P might come from. What relation might P have to surface tension, and to the average cross-section of the tube-like channels which conduct the water? Remind students of the formula for the surface tension of a bubble with internal pressure P (look in Physical Chemistry by Walter J Moore).

Further suppose that the flow dVdt in the tissue is given approximately by a simplified Poiseuille equation for viscous flow: dVdt = kp A dPdl, where kp is a constant and A is the cross-section of the tube-like channel.

Readers can work through the sums as an exercise and they will find the tissue clock equation: L = C(2 kp P t A), where t is the time, L is the length of pipe, P is the pressure, and A the area. It means that if the clock wet-edge moves 5cm in 1 minute, it will move 10cm in 4 minutes, 15cm in 9 minutes and so on.

The clock and 37 other suggestions can be found in Ink Sandwiches, Electric Worms by Neil Downie (Johns Hopkins University Press pound;33.50)


The tissue clock can be used by all ages. Older students can investigate it in more depth, looking at the maths or using it as an investigation for GCSE or A-level.


The experiment can be the basis for a science 1 investigation - eg when looking at solids, liquids and gases - and a test of evaluation skills. Why does the water not travel at a steady speed? Students might like to think about how the structure of particles affects a material's properties. How are the tubes similar and different to those in a plant's stalk?

KS 3-4

Talk about the shape of the meniscus in a tube and how pressure difference is related to surface tension. Ask students about the viscosity of different liquids and the change of viscosity with temperature.


Can students think of a practical application for the tissue clock?


KS 3-4

Discuss how to represent the data to understand what is going on.


Talk about differential and partial differential equations (see Mathematical Methods for Science Students by Geoffrey Stephenson).



Use a spreadsheet to model the behaviour of the water.


Graph the equation with a spreadsheet and compare a theoretical graph to the ideal curve. Bright students may be able to compose a finite difference equation which correctly models the clock.

More projects

Many more ideas are featured at where you can find practical tips, letters about projects and the science behind them, as well as information about Ink Sandwiches Electric Worms and Vacuum Bazookas, Electric Rainbow Jelly (Princeton University Press, pound;12.24), also by Neil Downie.


* Godel, Escher, Bach: The Eternal Golden Braid by Douglas Hofstadter (Penguin pound;16.99)

* Tables of Physical and Chemical Constants by George Kaye and THLaby (Longman)

* Physical Chemistry by Walter J Moore (Longman)

* Mathematical Methods for Science Students by Geoffrey Stephenson (Longman)

Some of the bits and pieces for doing the projects in the books are on sale by two UK firms:'snew.htm

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