the decrease in the value of emf of an unsaturated weston cell is about(a) 1µv per year(b) 1 mv per year(c) 5-10 µv per year(d) 30-50 µv per year

To determine the correct answer regarding the decrease in the value of the electromotive force (emf) of an unsaturated Weston cell, we can analyze the options based on standard values and characteristics associated with such cells.

1. Understanding Weston Cells: A Weston cell is known for its stable voltage output and is commonly used as a reference in measuring the voltage. However, its emf does decline over time.

2. Standard Values: The typical decrease in emf for an unsaturated Weston cell is reported in many sources to be around a few microvolts per year.

3. Analyzing Options:

   • (a) 1 µV per year: This value may be too low for a typical performance decline.
   • (b) 1 mV per year: This is significantly high for a Weston cell.
   • (c) 5-10 µV per year: This range aligns well with common observations.
   • (d) 30-50 µV per year: This is also an overestimate based on usual standards.

4. Conclusion: Based on the analysis, the option that best represents the typical decline in emf for an unsaturated Weston cell would be (c) 5-10 µV per year.

Final Answer

The decrease in the value of emf of an unsaturated Weston cell is about 5-10 µV per year (option c).