Minimumthermometer

1. The thermometric fluid

Unlike conventional mercury thermometers, these use almost exclusively mercury thermometers. Ethanol or another alcohol (often colored blue or red) is used. This has two reasons:

  • Freezing point: Alcohol has an extremely low freezing point ($approx. -114°C$), which allows measurements far below the freezing point of water.

  • Viscosity and wetting: Alcohol wets the glass wall differently than mercury, which is crucial for the movement of the drag pin (index).

2. The mechanism of the drag pin (index)

The core component is a small, dark glass rod located inside the capillary.

  • Cohesion and surface tension: As the temperature drops, the alcohol in the reservoir contracts. The meniscus (the curved surface of the liquid column) moves towards the reservoir. Due to surface tension, the meniscus acts like an elastic skin. When this "skin" meets the end of the drag pen, it exerts a force that pulls the pen along.

  • Asymmetrical resistance: As the temperature rises again, the alcohol expands. Since the pen is completely surrounded by the liquid and the adhesive forces between the glass pen and the capillary wall are sufficient to hold it in position, the alcohol simply flows around the pen without moving it.

3. Mathematical basis: Volume expansion

The change in column length in the thermometer follows the formula for linear or volumetric expansion:

Delta V = \gamma \cdot V_0 \cdot \Delta T

Where:

  • Delta V: the change in volume.
  • Gamma: is the coefficient of volumetric expansion of the fluid.
  • V_0 : is the initial volume.
  • Delta T: the temperature difference.

Since the cross-section of the capillary is constant, this change in volume leads to a precise linear shift of the meniscus – and thus of the index.

4. Sources of error and calibration

From a scientific point of view, there are two main sources of error with this instrument:

  1. Adhesion defects: If the thermometer cools down too quickly, some of the alcohol can stick to the glass wall ("tearing"), which artificially lowers the meniscus.

  2. Parallax: Since the index is located inside the glass, the reading must be taken at exactly a 90° angle to avoid refractive errors of the glass.