Notes on demonstrating
The pros and cons of the Whitley Bay smoke cell compared with observing very dilute milk
by David Walker, UK
Update November 2013: HD 1080P extended video clip of Brownian motion in dilute milk available to download. See below.
I first remember being shown Brownian motion in high school by the physics teacher using a microscope to observe smoke particles suspended in air in a glass cell. This is one of a number of recommended ways of demonstrating this classic experiment; others include observing aqueously suspended fine particles such as pigments (e.g. Gamboge, Indian ink) or fat globules in very dilute milk (refs. 1 - 3). Robert Brown studied the motion after whom it takes its name, in tiny particles released from the medium within pollen grains (not the pollen grains themselves which are too large to show the motion, ref. 4).
I was browsing eBay recently and noticed a used Brownian motion smoke cell which I purchased for a few pounds as was interested in comparing its effectiveness with other techniques tried; my preferred method as a hobbyist is to use extremely dilute milk as it is effective and fairly easy to set up. (See Micscape 'Home in close-up' article.)
cell purchased was the 'Whitley Bay' pattern made by Griffin &
George Ltd, which seems a popular
design and is still available new from educational suppliers. There has
been speculation online as to why it was apparently named after a small town on the
north east coast of England (ref. 5). Was the designer based there; or perhaps an
allusion of the smoke cell's lighting to the town's offshore lighthouse?
I'd be interested to hear from any reader who knows why.
Note added March 5th 2014: Thank you to a reader Albert who notes "It's called the Whitley Bay Smoke Cell because it was designed and invented by the physics department at Whitley Bay High School."
The cell is shown below. It is essentially a self contained lamp, lens and smoke cell to observe the bright smoke particles under a microscope against a dark background. It uses the 'festoon' type bulb* 12V 3W, to shine light through a solid glass rod to give a thin beam of light in the small cell. As smoke particles pass through this beam the particles are seen as white specks undergoing motion, both Brownian and other motion like convection. *The bulbs are readily available as they are used in e.g. some older car door courtesy lamps.
Cell with light shade off for
clarity. 1 - festoon bulb, 2 - solid glass rod acts as lens, 3 - small
glass cell sits in friction collar in base,
with cover slip, 4 - terminals for 12V supply.
Cell with light guard on and smoke cell removed and sitting on this cover to show the cell more clearly. This example has been well used as cell has tarry deposits in base. The instructions note the importance of regularly cleaning the cell walls.
Some practical disadvantages of both the design and its ability to unambiguously demonstrate Brownian motion were discussed by Marcus Rowland, a head science technician at a London school, in a 'New Scientist Forum' article 'The great Brownian motion swindle' (ref. 6), which prompted some letters from readers (ref. 7).
Having experimented with the cell for some time, the view of the smaller smoke articles brightly seen in motion against a black background is quite striking although simpler methods seem better at more clearly demonstrating Brownian motion.
Observations of the smoke cell design and its use:
There a number of resources online that describe the cell's use, e.g. on the 'Practical Physics' website.
It's suggested that paper drinking straws or corrugated paper is set alight then extinguished to 'pour' smoke into the cell; although the author rolled up some paper round a '00' paintbrush handle and taped in middle, then remove the 'former', and used this. Once the coverslip is put on, the smoke can be studied for some while, 5 - 10 mins at least from the author's trials.
A coverslip can easily slip off the top of the small cell, and the cell in the author's example was also not very tightly held at its base; disadvantages also noted in Rowland's article. A few tiny dabs of eg Vaseline on the cell's top face help prevent the coverslip shifting (not water which tends to cause condensation in the warm lighting.)
Tracing paper in plane of cell showing how the glass rod creates a thin beam of light. Graduations are in mm.
Using tracing paper, the lit area is about 4-5 mm below the top of the cell. This limits the objective used to those that have at least this working distance plus ideally a few more mm for safety. A 10x objective is recommended, and the author's Zeiss 10X NA0.22 has sufficient working distance, but the author had a preference for the Zeiss 6.3x NA0.16, as the wider field showed more particles and gave a safer working distance.
As the online resources recommend, starting with the objective by eye very close to the coverslip then focussing upward until the particles are seen works well.
The lighting is not that strong but the bright smoke particles can readily be seen by eye with typical 10x eyepieces; a slightly darkened room helps. To video the motion the author used a typical domestic C-mount monochrome security camera, sensitivity 0.05 Lux, which was just sensitive enough to record the particles in motion. Strong LED lighting is now available in the festoon bulb format for a few pounds, perhaps these bulbs may be more effective. Such lamps would also not emit infrared so may help minimise convection in the cell.
The random motion of the smaller smoke particles can be seen but the large volume of the cell relative to the particles and tungsten side lighting can introduce motion by convection as well. The narrow depth of field of the objective and tall cell also means that a given particle is only seen for a short time before they drift out of focus.
The advantages of studying very dilute milk:
From the author's past and more recent comparative trials using a drop of very dilute milk on a microscope slide with cover slip, this method does seems to offer a number of advantages to the smoke cell:
Apart from the microscope no special apparatus is needed, just a microscope slide, coverslip and some simple tools (see procedure below, click image).
If the coverslip is temporarily sealed along the edges with Vaseline, the initial currents die down in a few minutes and the fat globule motion by inspection is almost entirely Brownian.
The very thin liquid layer also ensures fat globules remain in focus, and the random motion can be studied more carefully.
The variation in sizes of the fat globules also allows the motion of smaller globules to be compared with that of larger globules.
Using normal transmitted light with the microscope allows much easier visual or video studies to be carried out.
Video comparison - smoke cell
cf dilute milk
Videos use Xvid compression, Codecs here if doesn't play on Windows / Linux PC.
If videos don't autoplay when click image, use the right mouse button to click image to 'save target' to PC, then view in PC's own player.
Camera: Hunt HTB-686D 0.05 Lux 580 TV line B/W security camera with Motic 16mm relay lens. A near infra red blocking filter was used,
and vital, as these sort of B/W cameras often don't have one. Video recorded to domestic DVD recorder. Then file edited in Virtual Dub.
Smoke cell, left 6.3x objective
with 5x eyepiece, right 10x objective with 10x eyepiece, (2 Mbyte and
1 Mbyte clips, 30 secs each).
In the author's trials, motion by convection varied depending on the focus plane, the above clips used focus planes to mimimise such motion.
A 10x objective is suggested in the maker's instructions but the depth of field becomes quite small with smoke particles not being observed for long before moving out of focus. A 6.3x objective was better in this respect; the lefthand clip above shows some motion by convection (along y axis in clip) in addition to Brownian motion.
Smoke was generated with coiled paper which gave particles of similar size, thus not allowing a study of the effect on Brownian motion on different sized particles. Fat globules in milk is better to demonstrate this aspect.
Very dilute milk in water: temporary sealed coverslip.
Left: brightfield, (2 Mbytes, 27 secs), optic mag 400x, (Zeiss 40x NA0.75 objective, 10x eyepiece.)
Right: darkfield, (8 Mbytes, 42 secs), optical mag 320x, (Zeiss 16x NA0.4 objective, 10x eyepiece, 2x Optovar.)
Motion by convection is virtually eliminated if the slip is sealed well and allowed to equilibrate for a few minutes; allowing Brownian motion to be more readily studied. The thin liquid layer keeps more fat globules in focus so can be viewed for longer. The difference in motion of larger and smaller fat globules can also be seen. To minimise the liquid thickness, the slip can be sealed by immersion oil instead of the thicker Vaseline.
Some experimentation is worthwhile with the milk dilution, too little and the fat globule density confuses the study (and gives glare in darkfield), too dilute and there's not a good range of particles. The concentration of the milk in above clip is a little on the high side for clear darkfield.
To study individual fat globules more closely eg for statistical analysis, inserting the C mount video camera directly into eyepiece with an adaptor introduces a useful extra mag caused by the small sensor crop.
Film clip details, 3 Mbytes, 10 secs, Cinepak Codec compression: Very dilute milk taken with a Russian 40X (N.A. 0.65) achromatic objective in bright field illumination on a LOMO Biolam. The image was projected into the video camera (Panasonic CL350 430 line resolution) without an eyepiece. The droplets range in size from about 0.5 to 3 µm (1 µm is 1/1000th of a millimetre). The milk dilution was about optimum, with a good range of different sizes to study individual particles without an excess cluttering the field. Video clip from an earlier article 'Microscopy around the home'.
Update 2013: HD 1080p footage (45 second) demonstrating Brownian motion in dilute milk added, reduced sized still shown right. Right mouse click this link to save the 250 Mb file in Quicktime mov format to PC. Do not directly click link to try to play such a large file in browser.
File details: Darkfield illumination, 40X objective with 10X eyepiece, total mag 400X. Camera Canon 600D. Horizontal field width 118 microns, typical fat globule size range 0.5 - 3 microns.
Free to use for personal and group educational purposes 'as is' or edited if credit to 'David Walker, Micscape'. Further distribution or commercial use not allowed without permission.
In transmitted brightfield, the effect is perhaps not as striking as the bright smoke particles against a dark background in the smoke cell, although if the milk is extremely dilute, darkfield can be used with the milk for a similar effect. Darkfield can be readily setup on many student microscopes likely to be in schools using a suitable patchstop in the condenser filter tray.
A Google search suggests that dilute milk or other aqueous suspended solids are widely used for the demonstration, including statistical studies of particle movement.
Smoke cell cf very dilute milk,
sumary of pros and cons.
(Personal view based on author's trials as a hobbyist who does not have firsthand experience of their use with students.)
Materials and cost
Cost significant if more than one for single or groups of students, ca £28 each in UK, $45 in USA. Each cell requires a power supply.
Cost minimal; microscope slides and coverslips, pipettes, milk, Vaseline. Existing microscope illumination can be used, natural or artificial lighting sufficient.
Skill required to setup
Can require some experience and dexterity depending on user's experience with smoke cell. Safe smoke sources required.
Can require some experience and dexterity depending on user's experience with microscopes. Darkfield requires more experience.
Ease of use on microscope
Requires flat open stage without clips. Other objectives on turret can interfere, ideally removed. More prone to objective / smoke cell damage due to small working clearances.
Dilution level of milk requires some experimentation, but solution could be prepared by demonstrator beforehand. Normal microscope working practices with microscope slides, coverslips and appropriate objectives.
Effectiveness at demonstrating Brownian motion.
motion and convection can be present.
convection, just Brownian motion if coverslip well sealed
and allowed to equilibrate. Stabler for longer periods than
Ease of video recording for demonstrations, studies of individual particles.
Light level low, requires sensitive camera. Objective mags above ca. 10x not possible because of working distance.
If brightfield used, plenty of light for video recording. Wider choice of objective allows higher mags if interested in monitoring individual particles e.g. for statistical analysis.
Comments to the author David Walker are welcomed.
References and resources
1) Google Book Search - early mentions of using gamboge suspended in liquids to study Brownian motion in 'The Treasures of Botany', 1866 and 'The Popular Science Monthly' 1915.
2) Google Book Search - early mention of using Indian ink to demonstrate Brownian motion in 'The Principles of Bacteriology a Practical Manual for Students and Physicians', by A C Abbott, 1915. (To distinguish this motion from that of live bacteria.)
3) Google Book Search - early observation that fat globules in milk undergo Brownian motion, 'Popular Science' magazine, Dec. 1876. vol. 10, no. 9. p139.
4) 'Brownian movement in Clarkia pollen: a reprise of the first observations' by Brian J. Ford. The Microscope, 1992, 40 (4), pp. 235-241. This fascinating paper describes how Robert Brown's observations were repeated using one of Brown's original microscopes. This demonstration was carried out to dispel the suggestion that Brown's microscopes were not good enough to show the motion. The paper also discusses many aspects of Brown's life and work. This paper is on Brian Ford's web site.
5) Whitley Bay, Wikipedia entry with photo of lighthouse.
6) 'The great Brownian motion swindle. Marcus L Rowland uncovers a case of mistaken identity.', New Scientist, Forum, 13 April, 1991, p. 49.
7) 'Browned off' - readers' letters (nos. 1, 3 and 5 in link list) in later issues of New Scientist in response to above article.
8) 'Brown's Brownian motion revisited' Science News, August 15, 1992. A recent controversy between scientists as to whether Brown observed the motion.
Published in the June 2009 edition of Micscape.
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