Leslie's Cube

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Description of the cube:

One of the traditional experiments to show how objects emit heat is "Leslie's Cube". This is a metal cube that has the vertical sides in four "colours". They are usually matt black, white, shiney (polished) and dull. Variants have gloss black and dull white. There is a hole at the top to accept water

image of Leslie's cube in action

Traditionally the cube was placed on a tripod and heated with a bunsen burner. As all walls are in contact with the water they should all reach the same temperature. Using a bunsen buner is not necessary, as a kettle can be used to fill the cube. This reduces the risk of corrosion.
It is suggested that a glass petridish be filled with a dozen or so 19mm bearings and marbles to create a turntable. If the cube is then placed on this, it can be easily rotated with a gloved hand. The distance from detector to cube face thus remains more constant if placed with care at the centre of the turntable.

CAUTION not all that is black is black.

Not all hot objects are ideal radiators. Some will have their radiation skewed to a particular part of the spectrum.
Optically black and white objects are hard to obtain. This is due to the eye's interpretation of pigment (See colour). Titanium oxide means that in some cases the black of a Leslie's cube may be optically whiter than the white. Candle black (soot) gives a good black and lead white paints are (optically) better than the safer, modern paints. If you are intending to repaint you cube it is worth looking at the spectral emissivity of the paint you use. The market has an optical black at (emissivity= 1.00) and a "plastic white" of around (emissivity=0.84). This is both safer and "whiter" than the lead based paint.

A thermopile (Moll's thermopile) is directed to each surface in turn at a fixed distance and the radiation reaching the thermopile noted. (A thermopile is a collection of thermocouples which gives a voltage depending on their temperature compared to the cold junction). The voltage output can easily be displayed on a digital multimeter or Voltmeter in the millivolt range.

If your school does not have access to a thermopile:

  • it is suggested that reasonable results may be obtained by placing four thermometers, held in retort stands/ clamps, at the same height/ position at about 5-10mm from each of the faces of the cube. [ The finer the range of the thermometer the better: one possible option if you have sufficient is to use clinical thermometers- having checked they have been re-set properly.[ Note: they will only show increases in temperature. However their range is restricted and may not prove as effective.] The important thing is the four thermometers must match exactly- same range, size, type, even manufacturer/ supplier. ]

Take an initial reading from each of the thermometers, then you carefully fill the cube with boiling water and replace the lid. then take readings from each of the thermometers, say every 30 seconds for five minutes- then plot the graphs.

  • If you have access to a datalogger with an Infra-red sensor, this is an obvious demonstration that can be applied.

The use of thermochromic strips is not likely to be successful, as the resolution at the transition temperature will not be suffient to show conclusive differences.

  • If your school has a "2cm microwave apparatus, this can be used as a radiation sensor (as part of the radiation is in the microwave region that the sensor is sensitive to). No power is applied and no transmitter should be used. It is simply placed across a sensitive (mV/microA)meter such as a digital multimeter or light spot galvanometer. It may even be used to show heat emitted from a human body practicalphysics.org link.

The use of a thermal camera will show the comparison quickly. A piece of masking tape applied across one edge and two walls will show the same reflectivity and confirms that the surface treatment is behind the different radiation characteristics. The image (below) is obtained from a FLIR I7 camera. The left hand side of the cube is matt black and the right hand side is polished copper.IRlesleyscube.jpg

It is recommended that the cube be dried thoroughly after use to prevent corrosion. Occasionally the seams may need attention. If the cube is brass this can easily be soldered.

Updated:--D.B.Ferguson 13:28, 29 January 2010 (UTC)

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