Since it is inconvenient to use the negative values when measuring low temperatures of either the Fahrenheit or the Celsius scales, in the nineteenth century, Lord Kelvin, an Englishman, invented an new temperature scale suitable for measuring low temperatures.
When a material is cooled, it looses heat, and its temperature decreases, until a point is reached where it has no more heat left to loose. At this point it is not possible to lower the temperature any further. This low temperature is called absolute zero. Lord Kelvin suggested that this absolute zero temperature be the basis of a new scale which begins with the value zero at absolute zero. At sea level on this scale, water freezes at 273 degrees, and boils at 373 degrees. So, just like the Celsius scale, there is a difference of 100 degrees between the freezing and boiling points of water. This new scale is called the Kelvin scale in honor of Lord Kelvin, and has the unit "Kelvin". To convert from degrees "Celsius" to degrees "Kelvin" use the formula:
Kelvin = degrees Celsius + 273
This scale is very convenient for recording the very low temperatures of liquid nitrogen and liquid helium. On this scale, nitrogen condenses to a liquid at 77 Kelvin, and helium, a chilly 4.2 Kelvin. YBa2Cu3O7 has a Critical Temperature of about 90 Kelvin, and BiSrCaCu2O9 a Critical Temperature of 110 Kelvin. Thus when they are immersed in liquid nitrogen at 77 Kelvin, they become superconducting.
Temperature can be accurately measured with thermometers designed and calibrated for use in the temperature range of interest. For all experiments in this guide using Colorado Superconductor Inc.'s family of superconductor kits, a range from room temperature to that of liquid nitrogen is of interest. Sophisticated thermometers typically do not operate over such a wide range. Thermocouple thermometers however are fairly accurate over this great a range of temperature variation.
A thermocouple consists of a mechanical junction of two dissimilar metals. This junction generates a small electrical potential (voltage), the value of which depends upon the temperature of the junction. Thus with calibration, and an appropriate choice of metals, one can obtain a thermometer for the desired temperature range. For our range (300 Kelvin to 77 Kelvin), a type T, or Copper-Constantan thermocouple is used.
The thermocouple junction has been carefully attached to the superconductors in our kits, and thermally balanced and calibrated. A simple digital milivoltmeter attached to the leads can be used to determine the voltage of this junction. Note that thermocouple leads must be connected to the voltmeter via wires of the same material and the junction to the thermocouple leads must be at room temperature. This voltage can be converted to the equivalent temperature with the help of the conversion chart included with each kit.