A beam balance is a device used for the determination of the mass of a body under gravitation. It consists of a beam supported at the centre by an agate knife edge resting on a support moving inside a vertical pillar. The beam carries a light pointer which moves over a scale. The beam also has two pans hanging from its ends, where the object and the standard weights are placed.

The principle of a beam balance is based on the law of moments, which states that when a beam is in equilibrium, the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about the same point. In other words, the product of the mass and the distance from the pivot point is equal for both sides of the beam.

To use a beam balance, the following steps are followed:

1. Place the beam balance on a level and stable surface.
2. Adjust the pointer to zero using the screw below the beam.
3. Place the object to be weighed on one pan and observe the pointer’s deflection.
4. Add standard weights to the other pan until the pointer returns to zero.
5. The mass of the object is equal to the total mass of the standard weights.

A beam balance is an accurate and reliable device for measuring mass. It has some advantages over other types of balances, such as:

• It does not require electricity or batteries to operate.
• It is not affected by temperature, humidity, or air pressure changes.
• It can measure large masses with high precision.

However, a beam balance also has some limitations, such as:

• It requires careful handling and maintenance to avoid damage to the knife edges and pans.
• It can be affected by external forces such as wind or vibrations.
• It can only measure mass, not weight, which is the force exerted by gravity on an object.

A beam balance is an example of a simple machine that uses a lever to create a mechanical advantage. It is one of the oldest and most widely used instruments for measuring mass.

There are several types of beam balance available, each with its own advantages and disadvantages. Some of the common types are:

• Double-pan balance: This type of balance has two pans that can be used to measure different weights simultaneously. It is useful for comparing the masses of two objects or for finding the mass of an object by using a known mass on the other pan. However, it is less sensitive and accurate than a single-pan balance.
• Single-pan balance: This type of balance only has one pan but is capable of measuring more precise weights. It usually has a counterweight that can be adjusted to balance the beam when an object is placed on the pan. It is suitable for measuring small masses with high accuracy.
• Triple-beam balance: This type of balance has three beams, each with a sliding weight that can be moved along a graduated scale. The sum of the weights on the three beams gives the mass of the object on the pan. It is widely used in laboratories and classrooms for measuring medium-sized masses with good precision.
• Roman steelyard: This type of balance has a single beam with a fixed weight at one end and a hook at the other end. The object to be weighed is hung from the hook and the beam is balanced by moving the fulcrum along a graduated rod. The position of the fulcrum indicates the mass of the object. It is an ancient device that can measure large masses with moderate accuracy.

Beam balances are used for various purposes, such as:

• Measuring the mass of substances in scientific experiments.
• Measuring the mass of food items in kitchens and markets.
• Measuring the mass of precious metals and gems in jewelry shops.
• Measuring the mass of luggage and cargo in airports and stations.