For a constant volume gas thermometer , one should fill the gas at

For a constant volume gas thermometer, one should fill the gas at a low pressure. This is because at low pressure, the gas behaves more ideally, meaning it follows the ideal gas law more closely. This ensures that the temperature readings from the thermometer are more accurate.

Here are the steps:

1. Ensure the thermometer is clean and dry before you begin.
2. Connect the thermometer to a source of the gas you are using.
3. Slowly open the valve to allow the gas to fill the thermometer. Monitor the pressure to ensure it remains low.
4. Once the thermometer is filled, close the valve and disconnect the gas source.
5. The thermometer is now ready to be used.

Remember, safety is paramount when handling gases. Always use appropriate protective equipment and work in a well-ventilated area.