What is electrical resistivity ? In a series electrical circuit comprising a resistor made up of a matallic wire, the ammeter reads `5 A`. The reading of the ammeter decreases to half when the length of the wire is doubled. Why ?

Explanation

### Step-by-Step Solution:

1. **Understanding Electrical Resistivity**:
- Electrical resistivity (ρ) is a property of a material that quantifies how strongly it resists the flow of electric current. It is defined as the resistance (R) of a material per unit length (L) and per unit cross-sectional area (A).
- The formula for resistance in terms of resistivity is given by:
\[ R = \frac{\rho L}{A} \]
...