Premium Membership is now 50% off. The colour lightens on exposure to air, and the properties of the wood are not seriously affected. 0000022407 00000 n Bacteria also appear during prolonged storage of wood in water, including seawater (e.g., in the case of old sunken ships). and Woods, S.D. Presently, wood has become an alternative to brick in the construction of homes. Degradation can affect wood of living trees, logs, or products, causing changes in appearance, structure, or chemical composition; these changes range from simple discoloration to alterations that render wood completely useless. Toxic extractives contained in wood are a delaying factor and are the main reason for differences in resistance to decay among species, but no wood is immune. Noteworthy is the spectacular decrease of electric resistance as moisture content increases from zero to the fibre saturation point. Practical use of the relationship of wood’s moisture content to its electric resistance is made in electric moisture meters. * hour * degF. trailer << /Size 814 /Info 770 0 R /Root 774 0 R /Prev 575159 /ID[<74a1ce985a79bed77dfa2070ced023e6>] >> startxref 0 %%EOF 774 0 obj << /Type /Catalog /Pages 769 0 R /CAPT_Info << /L [ (English US)] /D [ [ (English Science)()] [ (Default)()] ] >> /PageLabels 768 0 R /StructTreeRoot 775 0 R /Lang (en-US) /MarkInfo << /Marked true >> >> endobj 775 0 obj << /Type /StructTreeRoot /ParentTree 342 0 R /ParentTreeNextKey 204 /K 358 0 R /ClassMap 344 0 R >> endobj 812 0 obj << /S 622 /L 953 /C 969 /Filter /FlateDecode /Length 813 0 R >> stream Thermal conductivity is highest in the axial direction and increases with density and moisture content; thus, light, dry woods are better insulators. 0000010909 00000 n Conversely, when subjected to an electric field, wood exhibits mechanical deformation (changes in size). k is the heat transmitted through a 1-inch thickness of homogenous material per square foot per hour when there is 1 degree of temperature change. 0000001524 00000 n Wood exhibits the piezoelectric effect—that is, electric polarization (the appearance of opposite electric charges on opposite sides of a piece) occurs under mechanical stress. Normally, wood absorbs a very small portion of acoustic energy (3–5 percent), but special constructions incorporating empty spaces and porous insulation boards can increase absorption to as high as 90 percent. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Preservative treatment imparts considerable resistance to these organisms. Wood is subject to degradation by bacteria, fungi, insects, marine borers, and climatic, mechanical, chemical, and thermal factors. Large-dimension timbers (such as glued laminated beams) offer more resistance for a certain time, and fire-retardant treatments are also available. Thermal conductivity for common materials and products: Hardwoods (oak, maple..) Mineral wool insulation materials, wool blankets .. Softwoods (fir, pine ..) k is the heat transmitted through a 1-inch thickness of homogenous material per square foot per hour when there is 1 degree of temperature change. 0000008102 00000 n 0000003152 00000 n Thermal conductivity declines as the density of the wood decreases. These properties—dielectric constant and power factor—assume a practical importance in drying wood with electric current (a theoretical possibility, although not presently a reality), gluing wood with high-frequency electric current, or making electric meters (capacity and radio-frequency power-loss type) for measuring its moisture content. The pitch of sound produced depends on the frequency of vibration, which is affected by the dimensions, density, moisture content, and modulus of elasticity of the wood. Conductivity 3–21 Dielectric Constant 3–22 Dielectric Power Factor 3–22 Coefficient of Friction 3–22 Nuclear Radiation 3–23 References 3–23 he versatility of wood is demonstrated by a wide variety of products. Both walls have the same surface area and the same interior and exterior temperatures. The thermal conductivity of wood is relatively low because of the porosity of timber. However, because marble tiles are usually twice as thick as ceramic tiles, they offer greater protection against thermal conductivity. Black Friday Sale! Acting in combination with physical and chemical factors related to submersion, they can cause considerable structural changes, leading to breakdown of the wood after exposure to air. 0000020898 00000 n The flammability of wood can be reduced by chemical treatment (see the section Preservation). Wood is degraded or destroyed not with the passage of time but only under the action of external factors. When sound waves of extrinsic origin strike wood, they are partly absorbed and partly reflected, and the wood is set in vibration. Thermal conductivity declines as the density of the wood decreases. How thick would a solid concrete wall have to be to have the same rate of heat flow as an 8.0-cm thick solid wall of this wood? directly proportional to thermal conductivity inversely proportional to density of substance inversely proportional to specific heat all of the above ⇒ The highest thermal diffusivity is of. Important also are the dielectric, or poor-conductor, properties of wood. Tile floors, such as marble and ceramic, offer similar R-values when compared based on the measurement. 0000001171 00000 n This property makes wood suitable for heating purposes but is disadvantageous for its technical utilization. 0000024245 00000 n For musical instruments, a preference exists for selected spruce wood, but fir, pine, maple, and tropical woods also are used. Conditions of exposure are the same as for fungi—suitable temperature, moisture, and air. Wood must be raised to a temperature of about 250 °C (about 480 °F) for a spark or flame to ignite it, but at a temperature of about 500 °C (about 930 °F) ignition is spontaneous.

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