Ask A Scientist General Science Archive

Question:  How does a triple-beam balance work?
 rosalyn s strauss

Answer:
The name pretty much tells it all -- Beam & Balance.  A beam is balanced
on a knife-edge.  A sample to be weighed is placed in a pan on one end
and a known mass is placed on the other end.  The amount of the known
mass is varied by adding or taking away small (known) amounts until
the beam is balanced.  At this point the mass of the sample can be
determined from the required amount of known mass.
  The description above applies to a simple beam balance.  A triple-beam
balance incorporates two variations:  1)  The known mass is attached
to a lever and 2) there are three levers (or beams -- hence triple-beam)
with different masses on them (the masses are usually in the ratio of
1:10:100).  Remember that a lever can be used to increase the force
applied to a task (in this case, to lift the pan containing the sample).
By moving the known masses to different places on the lever we control
how much force the known mass applies to balancing the mass of the sample.
Usually the two heaviestknown masses have notches that they fit in at
(11) fixed positions along their beams.  The lightest mass slides on a
lever that is marked to indicate the amount of mass on the sample pan
that it will balance.
To use the triple beam balance place all the known masses to their "0"
positions (probably all the way to the left) and then put the sample in the
sample pan.  Move the heaviest known mass one position (make sure it fits
into its notch!) higher and see if the sample pan lifts. 

Repeat moving the mass until the sample pan DOES lift.  At this point
your known mass is too much, move the mass back one notch so that the
sample pan settles back down.
Repeat the same procedure with the next lighter known mass, moving
it up one notch at a time until the sample pan lifts, then moving
back one notch (it is very important that these two masses are always
squarely settled into their notches!).
Now, slowly move the lightest known mass until the pan lifts.  Continue
moving it until the balance pointer indicates you have balance.  Make
sure your are not touching the balance during this last step (i.e., move
the small known mass a little, take your hands away, then watch to see
if it is balanced.).
To determine the mass of your sample, take the notch number of the heavies
mass times it's mass (probably 100g), add the notch number of the middle
mass times it's mass (probably 10g), and add the number indicated by
the pointer on the lightest mass (1 to 10g).  (Note:  your balance
should indicate somewhere on the various beams what the masses of the
known weights are).
 
Beam balances are simple and reliable as well as being fairly easy to
use (with a little practice).  They have been around for thousands of
years -- a testament to their usefulness!
 gregory r bradburn