What is the principle of beam balance?
A beam balance measures mass as opposed to weight, so the mass
you weigh will be the same on the moon as it is on earth. Gravity
is taken out of the equation, unlike a spring balance that measures
weight and would measure an article to be 1/6 of the weight on the
moon as it would be on earth using the same spring balance that
relied on gravity.
The principle is that of moment, or turning force/torque),
calculated by force x distance. Fundamentally, in the case of a
balance beam, the force is gravity acting on each side of the
fulcrum of the balance, and distance is the distance from that
fulcrum.
Since gravity will be constant wherever you are, only moment or
torque will be relevant. A spring balance is not a comparison
technique, so gravity changes will be relevant to the result -
hence only weight can be measured.
Basically, if the two forces each side of the balance point
(fulcrum) are equal, the balance will be horizontal. The pointer on
the balance indicates this condition. The sample being weighed has
a specific mass generating a fixed moment at its fixed position.
The moment exerted by the mass on the other side of the fulcrum can
be varied according to the position of the sliding weights on the
beam, or lever.
These positions have been calibrated to correspond to specific
relative masses (popularly known as weights), so when each side is
balanced you can read the weight that is balanced against the
sample.
To be completely scientific, we are measuring torque when we use
a balance beam, and moment and toque each side will be equivalent
when the beam is balanced. Torque is a function of arm length and
applied force.
However, the point is that a balance beam measures true mass,
and not just weight that changes with changing gravitational force.
I hope this helps.