Triple Beam Balance
A triple beam balance measures mass by sliding three riders along calibrated beams until the pointer balances at zero.
Definition
A triple beam balance is a tool for measuring mass: the object sits on a pan, three sliding weights called riders move along three beams, and when the pointer lines up with the zero mark you add up the rider positions. The beams are typically graduated in $100\ \text{g}$, $10\ \text{g}$, and $1\ \text{g}$ steps, the last readable to $0.1\ \text{g}$, and because the balance compares masses rather than measuring the pull of gravity it reads the same in any location. Mechanically it works on the lever principle, the torque of the unknown mass about the fulcrum balanced by the torques of riders at known distances. Its accuracy depends on the zero adjustment, a level surface, clean knife edges, and riders fully seated, and unlike a spring scale it gives a true mass independent of $g$.
Example
To find a rock's mass, slide the $100\ \text{g}$ rider to $0$, the $10\ \text{g}$ rider to $10$, and the $1\ \text{g}$ rider to $3.5$: the mass is $0 + 10 + 3.5 = 13.5\ \text{g}$. Weighing salt for a dissolving experiment, place a weigh boat on the pan, zero the balance with the adjustment knob, and add salt until the riders read $5.0\ \text{g}$, moving the largest rider first and seating each one in its notch. Checking against a $50.00\ \text{g}$ reference, a balance reads $50.3\ \text{g}$; after the zero screw is corrected, ten readings of the same rock give $13.5\ \text{g}$ every time, and the balance is both precise and accurate to $\pm 0.1\ \text{g}$.
Key Insight
Always check that the pointer reads zero with nothing on the pan before you start, or every measurement will be off by the same amount. A rider not seated in its notch gives a wrong reading, and most classroom mass errors come from a misplaced $10\ \text{g}$ rider rather than from the balance itself. A balance cannot tell you whether it is accurate; only a known mass can, and verifying against a standard before a lab is the cheapest error prevention there is.