Mechanical balances, also known as mechanical weighing scales, function using a system of levers, springs, and balances to measure weight. The fundamental principle is to counteract the force of gravity acting on an object being weighed with a measurable mechanical force.
Here's a breakdown of how different types of mechanical balances work:
1. Spring Balances:
- Principle: Spring balances rely on Hooke's Law, which states that the force needed to extend or compress a spring by some distance is proportional to that distance.
- Mechanism:
- An object is attached to a spring (usually a helical spring).
- The weight of the object pulls down on the spring, causing it to extend.
- The amount the spring extends is proportional to the weight of the object.
- A calibrated scale indicates the weight based on the spring's extension.
- Example: A common bathroom scale that uses a dial or digital display to show weight.
2. Lever Balances (Equal-Arm Balances):
- Principle: Lever balances operate based on the principle of moments. A moment is the turning effect of a force. For a lever to be balanced, the sum of the clockwise moments must equal the sum of the anticlockwise moments.
- Mechanism:
- A beam is supported on a fulcrum (pivot point) in the center.
- The object to be weighed is placed on one side of the beam.
- Known weights are added to the other side of the beam until the beam is balanced (level).
- The weight of the object is equal to the sum of the known weights required to achieve balance.
- Example: Traditional laboratory balances where calibrated weights are used.
3. Double Pan Balances:
- Principle: Similar to lever balances, double pan balances rely on the principle of moments around a fulcrum.
- Mechanism:
- Two pans are suspended from a beam, which is supported by a central fulcrum.
- The item being weighed is placed on one pan.
- Standardized weights are added to the other pan until the beam is perfectly balanced.
- The weight of the object is equal to the sum of the standardized weights used to balance the scale.
- Example: Used for very accurate measurements, such as in chemistry labs for precise reagent weighing.
4. Counter Balances:
- Principle: Combines lever and sliding weight principles.
- Mechanism: A lever arm with a sliding weight. The unknown weight is placed on one end of the lever. The sliding weight is then moved along the arm until the lever is balanced. The position of the sliding weight indicates the weight of the object.
- Example: Balance scales often used in doctor's offices to measure a person's weight and height.
In Summary:
Mechanical balances work by using levers and springs to counteract the force exerted by gravity on an object. The balancing force is either directly measured (spring balances) or used to equate the unknown weight to a known weight (lever and double pan balances). These balances are typically durable and do not require a power source.
