Sec. 1-7] 



ME( 'HA NIC A L INPUT TRA NSDl 7 'l-JRS 



149 



same type of wood, for samples of different origin. The resistance of 

 wood also varies with the direction of the current; in the direction of 

 the grain the resistance is about 5^ of that in the direction perpen- 

 dicular to the grain. 



All electrical methods of moisture determination are strongly 

 influenced by temperature variation. In wood, an increase of tem- 

 perature from to 30°C reduces the resistance by a ratio 5 to 1 . 



The resistivity can be measured with d-c methods, the electrodes 

 being in direct contact with the test specimen, Fig. (1-7) 14a. Large 

 voltages (100 volts) are frequently 

 used, and polarization may be neg- 

 ligible. In some cases difficulties 

 arise in obtaining a reliable contact 

 between the measuring electrodes 

 and the specimen, in particular if 

 the specimen has a granular struc- 

 ture. It is then more advantageous 

 to measure the resistivity of the 

 sample in a capacitor, as in Fig. 

 (1-7)146, by a-c methods at audio- or 

 radiofrequencies. The equivalent 

 circuit of such a setup is shown in 

 Fig. (1-7)1 4rZ ; the variation with 

 moisture of the resistance R s or of the dielectric constant (capacitor 

 C s ) or of their combination (storage factor, dissipation factor, loss 

 factor, loss tangent) can be detected and measured with any bridge 

 arrangement. The presence of the capacitor C F in the circuit in 



10 



I10 10 



u 10' 



10' 



10 15 



Humidity, % 



20 



25 



Fig. (1-7)13. Resistance of wood be- 

 tween two electrodes plotted versus 

 the relative humidity (from J. Stanek, 

 Arch. tech. Messen, V 1281-5, (Nov- 

 ember, 1935; by permission). 



(b) (c) 



Fig. (1-7)14. Electrode arrangements for humidity determina- 

 tion in solids: (a) electrodes in contact with the test object; 

 (b) electrode separated by an air space from the object; (c) 

 insulated electrodes; (d) equivalent circuit; C F , capacitance 

 between electrode and sample; C s and R s , capacitance and 

 resistance of the sample. 



series with the specimen proper does not influence the result. Figure 

 (l-7)14eZ represents also the equivalent circuit for the frequently used 

 capacitor arrangement with insulated electrodes, Fig. (1-7) 14c, which 

 avoids a direct contact between the sample and the test electrodes. 



