.................. gives a constant voltage throughout its life.

Answer

**Step-by-Step Solution:**

1. **Identify the Type of Cell**: The question asks for a type of cell that provides a constant voltage throughout its life. We need to think about electrochemical cells and their characteristics.

2. **Consider Common Cells**: Common electrochemical cells include alkaline batteries, lead-acid batteries, lithium-ion batteries, and mercury cells. Among these, we need to determine which one is known for providing a constant voltage.

3. **Research on Mercury Cells**: The mercury cell is a type of electrochemical cell that is known for its stable voltage output. It uses mercury as one of its components and has a specific electrolyte.

4. **Understand the Electrolyte**: The electrolyte in a mercury cell is potassium hydroxide (KOH), which is a strong base. This electrolyte dissociates into potassium ions (K⁺) and hydroxide ions (OH⁻).

5. **Analyze the Reaction**: During the operation of the mercury cell, the chemical reactions do not consume the ions from the electrolyte. This means that the concentration of the ions remains constant throughout the life of the cell.

6. **Conclusion**: Since the concentration of the ions does not change, the voltage produced by the mercury cell remains constant throughout its life. Therefore, the answer to the question is that the mercury cell gives a constant voltage throughout its life.

**Final Answer**: Mercury cell gives a constant voltage throughout its life.

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