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Heat Conductivity of Wood 



It is a well-known fact that dry wood is a very poor conductor 

 of heat. This is well illustrated in the employment of wood for 

 mjitches and as handles for utensils and tools subjected to rather 

 high temperatures. Sawdust makes an excellent insulator against 

 changes in temperature. The same is true also of charcoal, which 

 is often used within the walls of ice chests, refrigerators, and 

 about pipes and tubing where a non-conductor of heat is required. 



A light, porous wood is a considerably poorer conductor of heat 

 than one that is dense, due to the greater amount of dead air 

 imprisoned in its cavities and to the less continuity of wood sub- 

 stance. Increase in the moisture content of wood is accompanied 

 by increase in conductivity. If the heat is great enough, a portion 

 of the contained water is converted into steam, which forces its 

 way through the wood, thereby raising the 

 temperature more quickly. 



It has been found by experiment that woods 

 are least conductive in a radial direction and 

 highest longitudinally. Tests can readily be 

 made by taking small boards of uniform di- 

 mensions, boring a hole through the center 

 of each for the introduction of a small steam 

 pipe or of a metal rod so arranged that it can 

 be heated. If the block is coated with a thin 

 layer of paratfine or wax the relative con- 

 ductivity of the wood may be observed from 

 the manner in which the paraffine or wax 

 melts. Instead of melting uniformly in a 

 circle with the pipe or rod as the center, the 

 actual shape of the melted portion is that of 

 an ellipse with its longest axis in the direc- 

 tion of the wood fibers, that is, longitudinally. 



The ratio of the longest and shortest axes 

 of the ellipse thus formed varies widely with 

 different woods. In lignum-vitje, black locust, 

 and Osage orange, for example, the ratio is 

 about 1.25 to 1 ; in oak, ash, maple and beech, 

 about 1.50 to 1 ; while in basswood, poplar, 

 magnolia and buckeye it is about l.SO to 1. 

 Thus the denser the wood the more nearly 

 does the shape of the ellipse approximate 

 that of a circle, or in other words, the more 

 nearly uniform is its conductivity of heat in all directions. Con- 

 versely, the lighter and less dense the wood the greater the con- 

 ductivity in longitudinal direction in proportion to that of the 

 radial and tangential. 



The question of the heat conductive power of wood is of im- 

 portance in determining the principles involved in kiln-drying 

 lumber. This very extonsi%'e business has developed along various^ 

 lines and is largely based upon rough empirical data. Too often 

 a person successful in handling one class of material feels that 

 he has the key to the entire situation. As a result, there are 

 many types of kilns and divergent views as to the best methods 

 of conditioning wood by artificial heat. 



It is well known that different kinds of wood and different 

 sizes of material require different treatment in order to secure 

 the best results. Undoubtedly the question of conductivity is 

 involved, though to just what extent is not yet fully known. In 

 large timbers considerable time is required to allow the heat to 

 penetrate to the interior. This is necessary if drying is to proceed 

 uniformly. If the outside dries too rapidly, a shell is formed 

 which is a much poorer conductor of heat than the interior, and 

 serves as an insulator against further entrance of heat. This dry 

 shell also resists the transpiration of the interior moisture and 

 retards the drying operation, besides causing severe strains in 

 the fibers. 



One advantage of beginning the drying of lumber in a moist 

 atmosphere is that the interior of the green lumber is heated with- 



out superficial drying taking place. If dry heat is applied, vapor- 

 ization begins at once from the surface of the lumber, and the 

 heat energy is utilized in this manner, instead of being first 

 conducted to the interior of the wood, as is necessary if the drying 

 is to proceed uniformly. 



In certain processes in the preservative treatment of timber 

 products penetration is secured by varying the temperatures of 

 the interior and exterior of the timbers. Thus when a stick of 

 wood is heated the air within it expands and a portion escapes. 

 If it is cooled by placing in a cold liquid, the air condenses and, 

 mowing to the unequal pressures within and without, the liquid 

 is forced in to take the place of the expelled air. The question 

 of conductivity is involved in determining the length of time a 

 timber should be exposed to external heat in 

 order to obtain the proper temperature in- 

 side. This will vary according to the size 

 and shape of the material, the density of the 

 wood, and the water content. 



The Forest Service has for several years 

 been conducting investigations of the prin- 

 ciples of drying lumber. A bulletin now in 

 press gives the results of a large number of 

 determinations of the specific heat of differ- 

 ent woods. By specific heat is meant the 

 ratio of the quantity of heat required to raise 

 a body one degree in temperature to the 

 quantity required to raise an equal weight of 

 water one degree. The specific heat of wood 

 is a measure of its heat capacity or the 

 amount of heat it will absorb or give out 

 in passing through a certain range of temper- 

 ature. It was found that the specific heat 

 of all woods is approximately the same, with- 

 out regard to species, and that the mean value 

 between freezing and boiling temperature is 

 0.3,27. S. J. K. 



LEAF PRINT OF RED HAW 

 (See page 25.) 



Increase in Per Diem Charge 



At a recent meeting of the American Kail- 

 way Association it was voted that after Janu 

 an- 1 a per diem charge of freight cars coming 

 under the jurisdiction of other roads than those which own the cars 

 shall be increased from thirty and thirty-five cents to one rate of 

 forty-five cents. 



The question before the vote caused somewhat heated arguments, 

 but was finally carried by a vote of 179 against 162. A most 

 significant fact and one which has most interesting bearing on 

 present traffic conditions is seen in the association records, which 

 reveal that the roads voting for the increase hold an aggregate 

 of 1,708,001 freight cars, whereas the roads representing the losing 

 side have an aggregate holding of only 638,885 cars. 



The significance of this condition lies in the possible effect it 

 will have in stimulating the roads to more general car building. 



A prominent railway publication says that it is common knowl- 

 edge that the lines having insufficient freight equipment are often 

 more willing to pay the additional small per diem charge than to 

 make such expenditures for new rolling stock. Whether or not 

 the new ruling will have the desired effect depends upon whether 

 the new rate is sufficient to prove more expensive to insufficiently 

 equipped roads than the actual building of increased equipment 

 of their own. 



While the wood-workers in the larger cities often can buy electric 

 power cheaper than they can operate power plants, the sawmill man 

 in the country town often can reverse the order and produce elec- 

 tricity for the town, thus finding a better use for his surplus fuel 

 than the waste burner. 



