Introduction:
A single column manometer is a device used to measure the pressure difference between two points in a fluid system. It consists of a U-shaped tube filled with a liquid, usually mercury, and a reservoir connected to one end of the tube. The other end is open to the atmosphere or the point where the pressure is to be measured. The liquid in the tube rises or falls depending on the pressure difference between the two points.

Explanation:
The purpose of using a large reservoir in a single column manometer is to negate the effects of change in level due to pressure variation. This is because when there is a pressure variation, the liquid level in the manometer tube changes. However, this change in level can be influenced by factors other than the pressure difference being measured, such as the density of the liquid and the length and diameter of the tube. By using a large reservoir, these effects can be minimized or eliminated, allowing for more accurate pressure measurements.

Effect of Pressure Variation:
When there is a pressure variation in the system being measured, it causes a corresponding change in the liquid level in the manometer tube. This change in level is proportional to the pressure difference, as given by the equation P = ρgh, where P is the pressure difference, ρ is the density of the liquid, g is the acceleration due to gravity, and h is the change in liquid level. However, the change in level can also be influenced by other factors, as mentioned earlier.

Minimizing the Effects:
By using a large reservoir, the effects of factors such as the density of the liquid and the length and diameter of the tube can be minimized. The large volume of the reservoir ensures that the change in liquid level due to pressure variation in the system is relatively small compared to the total volume of liquid in the manometer. This means that the change in level is less influenced by factors other than the pressure difference being measured.

Advantages:
Using a large reservoir in a single column manometer offers several advantages:
1. It improves the accuracy of pressure measurements by minimizing the effects of factors other than the pressure difference.
2. It provides a stable reference point for the liquid level, allowing for consistent and reliable measurements.
3. It reduces the influence of dynamic pressure variations due to motion, as the large volume of the reservoir helps to dampen these fluctuations.

Conclusion:
In conclusion, a large reservoir is used in a single column manometer in order to negate the effects of change in level due to pressure variation. By minimizing the influence of factors such as the density of the liquid and the length and diameter of the tube, the use of a large reservoir improves the accuracy and reliability of pressure measurements.