# Thermometricicities

A thermometric property is a physical property that changes measurably with temperature.

Thermometric properties

http://www.taftan.com/thermodynamics/TEMPERAT.HTM

Demonstration of some thermometric properties:

• length of liquid column, e.g. length of mercury column

as the temperature of the body increases so too does the length of a column of liquid in a thin capillary tube. Suitable liquids include mercury and alcohol. These

• emf of thermocouple

a thermocouple is made up of 2 different material conducting wires. These wires are in contact at 2 ends. When there is a difference in temperature between the 2 ends a small Potential difference (an e.m.f.) is created. This is due to each materials inheirant electron affinity properties. The size of this potential difference is proportional to the temperature difference between the junctions.

Thermocouples have very vast ranges, they are small and cheap to produce. However do not give an absolute reading of temperature, as they have to be referenced off a Cold Junction (a place with a known temperature)

quick responses

small

capable high temperature

no absolute value of temperature

Type K (chromel {90% nickel and 10% chromium}—alumel {95% nickel, 2% manganese, 2% aluminium and 1% silicon}) is the most common general purpose thermocouple

• resistance

Variation of the resistance of a metallic conductor with temperature

We just wound insulated copper wire around a cylinder (plastic pen-outer) about 100 times.Because it is insulated it wont short, and the plastic keeps it "tidy".

Remove 2cm of insulation from each end and attach croc clips.We leave them in the glycerol all year around, and cover them with clingfilm to keep them clean!

It works well. You will get a very small change in R (about 0.5ohms over 100degrees)

But I tell them that we want it like that otherwise the resistance of our house wiring would change from summer to winter, with all kinds of unwanted effects.

It costs next to nothing and every student can have/make their own one.

Resistance of a Metallic conductor Increases (↑) with Increasing temperature

Resistance of a Semiconductor (thermistor) Decreases (↓) with Increasing temperature

• pressure of a gas at constant volume

• volume of a gas at constant pressure

This might make more sense of the PV = k @ kT

http://id.mind.net/~zona/mstm/physics/mechanics/energy/heatAndTemperature/gasMoleculeMotion/gasMoleculeMotion.html

• colour.

It should be noted that 2 thermometers can not be expected to display exactly the same temperature as another thermometer (especially based on another thermometric property). This is because different thermometric properties do not change proportionally with the same change in degree of hotness

Thermometric properties can also be derived properties such as the bend of a bi-metallic strip.