394 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Choice pieces of wood of each of the seven kinds named in the 

 table would yield insulation resistances far higher than those given in 

 the column of averages, and not less than 1,000 megohms even in the 

 case of hard pine. 



TABLE II. 



Table II. gives the results of a great number of experiments upon 

 the electric resistances of slabs of different substances, throusfh which 

 currents were sent by means of mercury electrodes placed opposite 

 each other. Each electrode was contained in a cavity excavated in a 

 piece of ebonite (Figure 3), and was generally 

 about GO square centimeters in area. Compara- 

 tive measurements made with slabs of wood cut 

 across the grain were rather unsatisfactory, for 

 the reason that there seemed to be in every case 

 a resistance of contact ( Uebergangswiderstand) at 

 the common surface of the mercury and wood, and 

 the ma2:nitude of this resistance depended very much upon the rough- 

 ness or smoothness of the cut, and was sometimes greater than the 

 intrinsic resistance of the wood. In experiments made with currents 

 sent across the grain of the wood through slabs of different thick- 

 nesses cut from the same plank, the resistances seemed to follow 





Fig. 3. 



