62 ECONOMIC "WOODS OF THE UNITED STATES 



progress which should add materially to the present knowledge 

 of the subject. 



Reference 



TiEMANN, Harry D. : The Physical Structure of Wood in Relation to Its 

 Penetrability by Preservative Fluids, Bui. 120, Am. Ry. Eng. and 

 Maintenance of Way Ass'n, Jan. 1910. 



CONDUCTIVITY 



Dry wood is a very poor conductor of heat, as is well illus- 

 trated in its use for matches and as handles for utensils and tools 

 subjected to various temperatures. Increase in density or in 

 moisture content increases the conductivity of wood. Woods 

 are most conductive in direction parallel to the grain and least 

 so in radial direction, the ratio in some instances being as high 

 as 2 to 1. The difference between radial and tangential direc- 

 tions in this regard is slight, and is probably due to the fact that 

 in a tangential direction the bands of the denser and therefore 

 more conductive late wood are continuous, while radially they are 

 interrupted by alternate bands of the less dense early wood. 



Wood in a dry condition is a non-conductor of electricity. 

 Increase of water content reduces its value as an insulator. Light, 

 porous woods are more resistant to the passage of electric currents 

 than are dense woods; highly resinous woods, more than woods 

 without resin, since resin and oil are poor conductors of electricity. 



Wood is a good conductor of sound, particularly in a longi- 

 tudinal direction. The denser, the more uniform, and the dryer 

 the wood the greater is its ability to transmit sound. Unsound- 

 ness and decay materially reduce this property. 



References 



Gayer, Karl: Sehlich's Manual of Forestry, Vol. V, 1908, pp. 78-79. 

 ExNER, W. F.: Lorey's Handbuch der Forstwissenschaft, Vol. II, p. 117. 

 Mathey, Alphonse: Traits d'Exploitation Commerciale des Bois, Vol. I, 



Paris, 1906, pp. 63-65. 

 NoRDLiNQER, H. : Die Technischen Eigensohaften der Holzer, pp. 56-114. 



RESONANCE 



"If a log or scantling is struck with the ax or hammer, a 

 sound is emitted which varies in pitch and character with the 



