1G4 EXPERIMENT STATION. [Jan. 



The resistance, however, may equal 2.5,000 ohms more or 

 less, in 10 feet of the trunk of an elm or maple tree. This con- 

 stitutes a comparatively high resistance. The resistance of the 

 sapwood is very much greater, and probably that of the heart- 

 wood is even higher than that of the sapwood. 



In determining the electrical resistance it is necessary to 

 know the path or course of the current, and the only manner in 

 which the electrical resistance of different tissues can be deter- 

 mined accurately is by isolating the tissues. B}^ girdling a tree 

 and scraping the trunk down to the solid wood we can get the 

 resistance of the wood. Mr. Chapman found the resistance of 

 a freshly cut rock maple stem, II/2 inches in diameter, to be 

 70,000 ohms when intact, i.e., with the bark on, but 150,000 

 ohms when the bark was removed. The electrodes were 1 foot 

 apart. 



Some experiments which have been made indicate that next 

 to the cambivim the phloem has the least resistance, followed by 

 the sapwood. The outer bark appears to offer the most resist- 

 ance, but when this is moist, as during rain storms, the resist- 

 ance may be somewhat decreased. When leakage occurs, owing 

 to groimding of the electric currents from high tension wires in 

 moist weather, burning results, but this is due to the presence 

 of a film of water on the bark, and what is termed '' arcing " 

 occurs. The resistance obtained from an elm tree, with the 

 electrodes 10 feet apart and in contact with the cambium, was 

 10,098 ohms, whereas when the electrodes were inserted into 

 the middle of the cortex or phloem we obtained 11,300 ohms 

 resistance. When driven ^ inch into the wood the resistance 

 was 98,700 ohms. The outer bark gave 198,800 ohms resist- 

 ance, but when the electrodes were inserted slightly deeper 

 into the bark we obtained 109,900 ohms. It must not be under- 

 stood, however, that these readings gave the electrical resistance 

 of 10 feet of the various tissues enumerated except in the case 

 of the cambium, since if these tissues were isolated the resist- 

 ance would be much gi*eater. They show that there is much 

 difference in the resistance of different tissues, but in all cases 

 here we obtained merely a resistance of the cambium, together 

 with that of a part of the other tissues, which the current had 

 traversed from its various points of entrance to the cambium. 



