Which of the following is a non-ohmic material ?

Options

A. Carbon
B. Vacuum Tubes.
C. Silver
D. Iron

Explanation

To determine which of the following materials is non-ohmic, we need to understand the concept of non-ohmic materials and Ohm's Law.

### Step-by-Step Solution:

1. **Understand Ohm's Law**:
- Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points, provided the temperature and material remain constant. Mathematically, it is expressed as:
\[ V = IR \]
- Here, R is the resistance, which remains constant for ohmic materials.

2. **Identify Non-Ohmic Materials**:
- Non-ohmic materials do not follow Ohm's Law. This means that the current does not increase linearly with an increase in voltage. Common examples of non-ohmic materials include diodes, transistors, and vacuum tubes.

3. **Analyze the Given Options**:
- **Carbon**: Generally behaves as an ohmic conductor under certain conditions but can exhibit non-ohmic behavior under others (e.g., in thermistors).
- **Vacuum Tube**: This is a device that allows current to flow in a vacuum between electrodes. It does not follow Ohm's Law and is considered a non-ohmic material.
- **Silver**: A good conductor that follows Ohm's Law, hence it is an ohmic material.
- **Iron**: Also a conductor that generally follows Ohm's Law under normal conditions.

4. **Conclusion**:
- Among the options provided, the vacuum tube is the only non-ohmic material since it does not obey Ohm's Law.

### Final Answer:
The non-ohmic material among the given options is the **vacuum tube**.