Which one of the following is not made of soft iron?

Options

A. Electromagnet
B. Core of transformer
C. Core of dynamo
D. Magnet of loudspeaker

Explanation

To solve the question "Which one of the following is not made of soft iron?" we will analyze each option provided:

1. **Electromagnet**:
- Electromagnets are typically made of soft iron because soft iron has high magnetic permeability and can be magnetized easily. This allows the electromagnet to generate a strong magnetic field when an electric current passes through it.

2. **Core of Transformer**:
- The core of a transformer is also made of soft iron. The soft iron core helps in efficiently transferring magnetic flux between the primary and secondary coils, minimizing energy loss.

3. **Core of Dynamo**:
- Similar to transformers, the core of a dynamo (or generator) is made of soft iron. This is to ensure efficient magnetic field generation and energy conversion.

4. **Magnet of Loudspeaker**:
- The magnet used in a loudspeaker is typically a permanent magnet, not soft iron. Permanent magnets are made from materials like ferrite or neodymium, which maintain their magnetic properties without the need for an external electric current.

Based on the analysis above, the correct answer to the question is:

**Magnet of Loudspeaker** (this is not made of soft iron).

### Step-by-Step Solution:
1. **Identify the materials used in each option**:
- Electromagnet: Made of soft iron.
- Core of Transformer: Made of soft iron.
- Core of Dynamo: Made of soft iron.
- Magnet of Loudspeaker: Made of permanent magnet.

2. **Determine which option does not use soft iron**:
- The magnet of the loudspeaker is the only option that is not made of soft iron.

3. **Conclude the answer**:
- The option that is not made of soft iron is the **Magnet of Loudspeaker**.