A balance is a scientific instrument used to determine the mass of an object. Mass is a measure of the amount of matter in an object, which remains constant regardless of its location in the universe. This differs from weight, which is the force exerted on an object due to gravity and changes depending on the local gravitational field, such as on the moon versus Earth. Laboratory balances are designed to compare the unknown mass to a known standard.
Principles of Mass Measurement
The fundamental concept allowing all balances to function is the principle of equilibrium. This principle is applied by comparing the gravitational force exerted by an unknown mass to the force exerted by a known mass or a calibrated counter-force. While a spring scale measures weight by sensing the downward pull of gravity, a balance determines mass by this comparative method.
The operation of a balance is rooted in the law of the lever. This law dictates that a lever is in equilibrium when the forces and their distances from the central pivot point, or fulcrum, are equal. When an object is placed on the balance, the downward force of gravity acting on its mass creates a turning force, or moment, around the fulcrum. The balance measures the unknown mass by generating an equal and opposite moment, whether through physical counterweights or electronic means. This comparison ensures the measurement is independent of minor variations in the local gravitational field.
How Mechanical Balances Achieve Equilibrium
Traditional mechanical balances, such as the two-pan balance or the triple-beam balance, rely entirely on physical comparison to determine mass. These instruments utilize a first-class lever, where a central fulcrum supports a beam with a weighing pan at one end and a system for applying known masses at the other. When an unknown sample is placed on the pan, the beam tilts downward due to the gravitational force acting on the object.
To restore equilibrium, known standard masses or sliding weights called riders or poises are manually adjusted on the beam. The mass of the object is determined when the beam is perfectly level, indicating the downward force of the unknown mass is exactly balanced by the known masses. In an equal-arm balance, the distance from the fulcrum to the pan and to the counterweights is identical, meaning the masses themselves must be equal for the system to balance. A triple-beam balance uses a set of beams with specific riders that can be placed in calibrated notches, allowing for a precise, cumulative reading of the unknown mass.
The Technology Behind Digital Balances
Modern digital balances, including high-precision analytical models, replace the mechanical comparison of masses with an electronic force measurement. The primary component in many digital scales is a load cell, which converts the force exerted by the object into an electrical signal. Within the load cell, strain gauges are thin, flexible sensors bonded to a metal structure that deform slightly under load. This deformation causes a change in the electrical resistance of the strain gauge, which is then measured and translated into a mass reading displayed on the screen.
For instruments requiring the highest resolution, such as laboratory analytical balances, the principle of electromagnetic force restoration (EMFR) is employed. When a sample is placed on the pan, the downward gravitational force causes a slight vertical displacement. A coil attached to the weighing pan is suspended within a permanent magnetic field. An electronic sensor detects this minute movement and immediately sends an electric current through the coil to generate an upward electromagnetic force.
This counteracting force restores the pan to its original, or null, position, effectively “weighing” the object electronically. The amount of electrical current required to generate this restorative force is directly proportional to the object’s mass. This current is measured and converted into a digital mass value. EMFR systems offer superior stability and repeatability because the pan is maintained in a nearly motionless state, reducing the influence of mechanical friction or creep.
Maintaining Accuracy in Measurement
Regardless of the technology used, a balance’s accuracy depends heavily on proper maintenance and control of the operating environment. Calibration is the process of adjusting the balance to ensure that the mass readings align with certified reference weights. Balances require regular calibration using external reference weights or internal automated features. Before any sample is added, the balance must also be zeroed, or tared, to establish a true starting point and account for the weight of any container used.
Environmental factors can introduce significant errors, especially in high-precision weighing. These factors include:
- Air drafts from ventilation systems or open doors, which exert measurable pressure on the weighing pan and are often mitigated by an enclosed draft shield.
- Temperature fluctuations, which can cause internal electronic components to expand or contract, affecting sensitivity.
- Vibrations from nearby equipment or foot traffic, which destabilize the mechanism and require specialized anti-vibration tables.
- Static electricity, which causes lightweight samples to cling to the pan or container, requiring antistatic devices or humidification.