How much heat it holds
Density ρ and specific heat c describe how much energy is needed to warm a volume of material.
volumetric capacity = ρ × cThermoreception · heat transfer
Nine degrees under your fingertip
Aluminium and wood left in the same room can both sit at 20 °C. In the first instant, however, your skin meets them at roughly 20.6 °C and 29.2 °C.
The feeling of cold tracks how quickly heat leaves your skin. Aluminium exchanges heat with skin far more intensely than wood even though a thermometer gives both objects the same reading.
Put your fingertip in the modelOne thermometer, two sensations
When your fingertip touches a cooler object, the skin surface and material surface move towards a shared temperature. Its position depends on thermal effusivity.
How readily it moves
Conductivity k describes how readily thermal energy spreads through the material.
effusivity b = √(k × ρ × c)Who pulls the interface
The material with greater effusivity draws the contact temperature closer to its own. Modelled aluminium has roughly 67 times the effusivity of the representative wood.
baluminium / bwood = 66.95The contact lab
Choose two materials and their starting temperatures. The result is the idealised interface temperature immediately after touch.
Contact difference8.7 °C
Effusivity ratio67×
The approximation assumes uniform surfaces, perfect contact and bodies thick enough for the short interval being modelled. It shows the direction and scale of the effect without predicting every real touch exactly.
Measure it with your own skin
You need two dry objects that have rested together in the same room: one metal and one wood. A room thermometer can confirm their shared environment.
Touch the metal and wood simultaneously with the pads of your index fingers. Attend to the first impression.
Swap hands between the objects and repeat. Reversing them reduces the natural differences between fingertips.
Then maintain contact. The initial contrast should fade as the local temperatures evolve.
The numbers behind the sensation
The model combines published thermal properties for skin, aluminium and a representative wood with the classical formula for contact between two semi-infinite bodies.
Interface temperature
Tcontact = (bobject × Tobject + bskin × Tskin) / (bobject + bskin)Effusivity b measures a surface's ability to exchange heat during transient contact: b = √(k × ρ × c).
| Material | Conductivity k | Effusivity b |
|---|---|---|
| Skin | 0.372 W/(m·K) | 1,183 J/(m²·K·√s) |
| 1060 aluminium | 234 W/(m·K) | 23,868 J/(m²·K·√s) |
| Representative wood | 0.140 W/(m·K) | 356 J/(m²·K·√s) |
Wood is evaluated at a relative density of 0.50 and a moisture content of 12%. The USDA handbook warns that actual conductivity values may differ by as much as 20%.
What the model can tell us
The calculation isolates the mechanism that dominates the first thermal impression. Experimental research confirms the role of effusivity and identifies where living skin becomes more complicated.
The contact temperature follows from energy conservation between two idealised bodies, each treated as semi-infinite. For skin 2.5 mm thick, the approximation remains suitable for roughly the first 3 seconds. Beyond that interval, tissue and object depth matter increasingly.
During real touch, microscopic contact resistance depends on pressure, roughness and moisture. Blood flow, metabolism, object shape and material thickness shift the temperature away from the ideal value.
Experiments synthesised in the research literature found that participants reliably separated materials whose effusivities differed by a factor of at least 3 under the tested conditions. The modelled aluminium-to-wood ratio here is 66.95.
- Interpretation: the simulator compares the likely intensity of the first thermal impression.
- Limit: the calculated result is not a temperature measured on the reader's fingertip.
- Use: the tool is educational and does not assess nerve sensitivity or circulation.