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Science Forum Index » Chemistry Forum » ISTEP microbalance
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| Author |
Message |
| Allan Adler |
 Posted: Mon Jan 05, 2004 9:46 am |
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Guest
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In vol.III of the book, Guidebook to Constructing Inexpensive Science Teaching
Equipment, published by the Inexpensive Science Teaching Equipment Project
(ISTEP) at the University of Maryland, they describe the construction of
what they call a microbalance. It is allegedly sensitive enough to weigh
one human hair or one mosquito (I don't know what these weigh, nor what
fraction of a human hair or of a mosquito the apparatus will also weigh).
The construction is given on p.22 of Vol.III (vol.III is entitled "Physics",
but the equipment is used in vol.II as well, which is entitled "Chemistry").
Unfortunately, the necessary diagrams are incomplete and I'm not sure I
understand all details of the construction this apparatus.
Here is what one uses:
(1) For the support
(A) a block of wood, 15 cm x 2 cm x 2 cm
(B) 1 screw, 1 cm long
(C) 1 aluminum sheet, 6 cm x 2 cm x 0.02 cm
(2) For the balance arm
(D) 1 straw, about 16 cm long
(E) 1 bolt, about 1 cm long
(F) 1 needle, 3 cm long
(3) For the scale
(G) 1 wood strip, 12 cm x 2 cm x 0.4 cm
(H) 1 piece of white paper, 10 cm x 2 cm
(I) 2 rubber bands
No diagram is given for the scale (3), but it is easy to guess how one is
supposed to attach (G) to (A) and then to use (I) to hold (H) on (G).
Apparently one is supposed to write calibration marks on the paper after
weighing fractions of sheets of paper whose total weight is somehow known.
The construction of the support (1) is pretty clear: one bends (C) in two
places so that it consists of three 2 cm x 2 cm squares shaped like a
letter C lying on its on its back. A hole is drilled in the middle square
so that the C can be screwed (B) to the wooden block (A), with the two
squares on the end along the long sides of the block.
The diagram for (1) is completely clear and adequately labeled with reference
to the parts list. For the balance arm (2), there is a drawing of the straw
and its "scale pan", but no labels referring to the parts list. Here are
the instructions: "Take the straw (D) and select a short bolt (E) which
fits into the end of the straw. Screw the bolt partway into the straw.
Cut the free end of the straw with a pair of scissors to make an appropriate
scale pan in the balance arm. Pierce the straw near to the top surface,
and sufficiently close to one end, with the needle (F) so that the latter
will serve as a pivot. Balance the straw on the support. A few trials will
be necessary to obtain a suitable position for the needle."
It seems that the scale pan is made by cutting the end of the cylindrical
straw down to a semi-cylinder. That seems to invite the risk of stuff
falling down the straw when it is weighed, especially when one is weighing
finely ground material such as dust, thereby ruining the calibration of the
instrument. As for piercing "the straw near to the top surface", that
presumably refers to the upper semi-cylinder and the appropriate end to
do it is presumably near the bolt. Where do the ends of the needle go?
Maybe they simply rest on the two vertical squares of the aluminum (C).
Another possible flaw in the construction is that the straw will bend a little
under the weight. Admittedly that is also true of metal balances, but I
would expect it to be more pronounced in a straw. Still, it seems like
a nice, simple and perhaps surprisingly useful piece of improvised
apparatus.
Since the book was written, a lot of other resources have become more
widely available which might improve the design and its accuracy or
calibration without substantially increasing its near zero cost. Any
suggestions on how one might take advantage of such resources would
be welcome.
Ignorantly,
Allan Adler
ara@zurich.ai.mit.edu
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* *
* Disclaimer: I am a guest and *not* a member of the MIT Artificial *
* Intelligence Lab. My actions and comments do not reflect *
* in any way on MIT. Moreover, I am nowhere near the Boston *
* metropolitan area. *
* *
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