**In Millikan's oil drop experiment, the oil droplets acquire a negative charge.**

**Explanation:**

The oil drop experiment, conducted by Robert A. Millikan in 1909, was a groundbreaking experiment in the field of physics that determined the charge of an electron. The experiment involved suspending tiny oil droplets in an electric field and observing their behavior. Here's a detailed explanation of why the oil droplets acquire a negative charge:

**1. Electric Field Setup:**
- The experiment begins by creating a uniform electric field between two charged plates.
- The plates are connected to a power supply, which applies a potential difference (voltage) across them.
- The electric field in this setup is directed from the positively charged plate (anode) to the negatively charged plate (cathode).

**2. Introduction of Oil Droplets:**
- A fine mist of oil droplets is sprayed into the chamber between the two plates.
- Due to gravity, some of these droplets settle down while others remain suspended in the air.
- Millikan used an atomizer to produce very small oil droplets, ensuring their size was in the range of micrometers.

**3. Observing the Droplets:**
- A microscope is used to observe the oil droplets suspended in the chamber.
- By carefully adjusting the electric field and the voltage, Millikan was able to balance the gravitational force acting on the droplets with the electric force.
- When this balance is achieved, the droplets move neither upward nor downward and remain stationary.

**4. Determining the Charge:**
- Millikan observed that the suspended oil droplets acquired either a positive or negative charge.
- By adjusting the voltage, he could measure the electric force acting on the droplets.
- By measuring the velocity of the droplets when they were balanced, he could calculate the electric force and equate it to the gravitational force.
- Using this data, Millikan was able to calculate the charge on each droplet.

**5. Charge of the Oil Droplets:**
- The oil droplets in Millikan's experiment acquire a negative charge.
- This was primarily due to the fact that the oil used (usually oil of density less than water) had a lower ionization energy than the surrounding air.
- As a result, when the droplets were exposed to radiation or cosmic rays, they gained excess electrons from the surrounding air.
- The excess electrons caused the droplets to be negatively charged.

In conclusion, in Millikan's oil drop experiment, the oil droplets acquire a negative charge. This is because the oil used has a lower ionization energy than the surrounding air, causing the droplets to gain excess electrons and become negatively charged.

-ve that's why it was used to determine the charge on electron