Move the rider, then take your hands off and watch the beam until it stops.
01Check what is on the beam and where the pivot is before you touch anything.The balance compares two moments about the knife edge. Which mass is where decides which way the beam will go when you move the rider.
02Slide the rider to roughly where you expect the balance, then let go completely.Moving it while the beam swings makes it impossible to tell which change caused what. One deliberate adjustment at a time.
03Take your hands off and watch the pointer through several swings.The swings are the beam losing the energy it arrived with. Until they have died away, its position is a moment in a journey rather than a result.
04Wait for it to come to rest, and only then look at where the pointer sits.A beam settles to the angle at which the torques balance. That angle is the measurement; anything read earlier is a different number that happens to be on the same scale.
05Note which side it rests on and how far over, and move the rider accordingly.A large tilt means move a long way, a small one means you’re close, and the side tells you the direction. Use it and you converge in three or four adjustments instead of a dozen.
06Never hold the beam still or press the pointer to the centre.It’s the commonest way this practical is spoiled, and it destroys exactly the information you’re trying to collect. If the beam won’t settle, find out what is disturbing it.
07Read the rider’s position with your eye square to the beam’s scale.The rider stands clear of the graduations, so a reading taken from one side is a parallax error on top of a balance you took trouble over.
Laboratory
Slide the rider somewhere and then leave it alone. Watch the pointer: it will swing past its resting place more than once before it stays there.