Air is a bad conductor of heat or partly conducts heat. Still, a vacuum is to be placed between the walls of the thermos flask because:
| 1. | it is difficult to fill the air between the walls of the thermos flask. |
| 2. | due to more pressure of air, the thermos can get cracks. |
| 3. | by convection, heat can flow through the air. |
| 4. | on filling the air, there is no advantage. |
While measuring the thermal conductivity of a liquid, we keep the upper part hot and the lower part cool, so that:
| 1. | convection may be stopped |
| 2. | radiation may be stopped |
| 3. | heat conduction is easier downwards |
| 4. | it is easier and more convenient to do so |