Thermal diffusivity is a combination of physical properties (conductivity, density and specific heat capacity) (λ/ρc_{p}) denoted by κ, naturally arising in the derivation of the *conduction equation* and having physical significance in the context of transient conduction processes. It may be regarded as the ratio of the Thermal Conductivity of a material to the specific heat capacity of that material. Thus copper, with a high value of κ (= 11 × 10^{−5} m^{2}·s^{−1}), has a high conduction rate relative to its heat storage capacity and so responds quickly to changes in temperature. Conversely insulating materials may have κ values 100 times smaller, these low κ values being a measure of the high thermal inertia of such materials. Values of κ are often calculated from the widely available data on λ, ρ and c_{p} but directly determined values are also available. Data values can be found in Touloukian and Ho (1977), an extensive data bank compiled by the Thermophysical Properties Research Center at Purdue University and continually extended and updated by CINDAS at the same university. Other readily available data books are those by Kaye and Laby (1986) and Perry (1984). Extensive data banks are also available in computerized form and as CDROMs. Experimental methods for the determination of thermal diffusivity and conductivity of solids, liquids and gases are described by Tye (1969). κ values for gases show a strong dependence on temperature and pressure; methods for estimating κ values at temperature /pressure values outside tabulated ranges are described by Reid et al. (1987).

#### REFERENCES

CINDAS, Center for Information and Numerical Data Analysis and Synthesis, Purdue University, 2595 Yeager Road, West Lafayette, IN 47906, USA.

Kaye, G. W. C. and Laby, T. H. (1986) *Tables of Physical and Chemical Constants*, 15th edn., Longmans Scientific and Technical, Harlow, UK.

Wakeham, W. A., Nagashima, A. and Sengers, J. V. (1991) (Eds.) *Experimental Thermodynamics*, Vol. Ill, Chapters 6, 7 and 8.

Millat, J., Dymond, J. H. and Nieto de Castro, C. A. (1996) (Eds.) *Transport Properties of Fluids: Their Correlation, Predictions and Estimates*, Cambridge University Press, New York.

Perry, R. H. and Green, D. W. (1984) *Perry's Chemical Engineers' Handbook*, McGraw-Hill, New York.

Reid, R. C., Prausnitz, J. M. and Poling, B. E. (1987) *The Properties of Gases and Liquids*, 4th edn., McGraw-Hill, New York.

Touloukian, Y. S. and Ho, C. Y. (1977) (Eds.) Thermophysical Properties of Matter, *The TPRC Data Series* (13 volumes), Plenum Press, New York, 1970 to 1977.

Tye, R. P. (1969) *Measurement of Thermal Conductivity*, Vols 1 and 2, Academic Press, New York.

#### References

- CINDAS, Center for Information and Numerical Data Analysis and Synthesis, Purdue University, 2595 Yeager Road, West Lafayette, IN 47906, USA.
- Kaye, G. W. C. and Laby, T. H. (1986)
*Tables of Physical and Chemical Constants*, 15th edn., Longmans Scientific and Technical, Harlow, UK. - Wakeham, W. A., Nagashima, A. and Sengers, J. V. (1991) (Eds.)
*Experimental Thermodynamics*, Vol. Ill, Chapters 6, 7 and 8. - Millat, J., Dymond, J. H. and Nieto de Castro, C. A. (1996) (Eds.)
*Transport Properties of Fluids: Their Correlation, Predictions and Estimates*, Cambridge University Press, New York. - Perry, R. H. and Green, D. W. (1984)
*Perry's Chemical Engineers' Handbook*, McGraw-Hill, New York. - Reid, R. C., Prausnitz, J. M. and Poling, B. E. (1987)
*The Properties of Gases and Liquids*, 4th edn., McGraw-Hill, New York. - Touloukian, Y. S. and Ho, C. Y. (1977) (Eds.) Thermophysical Properties of Matter,
*The TPRC Data Series*(13 volumes), Plenum Press, New York, 1970 to 1977. - Tye, R. P. (1969)
*Measurement of Thermal Conductivity*, Vols 1 and 2, Academic Press, New York.