ELECTRICAL INJURIES TO TREES. 6 



It will be noted that these resistances were taken on the east, south, 

 Avest and north sides of the trees, and represent averages of weekly 

 obsei-vations. The lowest resistance (data not given in the table) ob- 

 tained from the majDle was 14,000 ohms, and the highest, 33.000 ohms. 

 In the case of the elm the lowest resistance was 6,300 ohms, while the 

 highest was 29,400 ohms. These resistances are relatively low, for in 

 cold weather they often exceed 100,000 ohms. The lower resistance in 

 all eases corresponds to periods of high temperatures, and the highest 

 to periods of the lowest temperature. The ditTerence shown by the 

 various sides of the tree is also related to temperature. 



As might be expected, there is considerable difference in the electrical 

 resistance of various trees as well as of the different tissues found in 

 trees. The heartwood, sapwood, cambium, bark and sieve tubes j^ossess 

 quite different properties and functions, and their electrical resistance 

 would naturally vary to a large extent. The living cells containing 

 protoplasm, such as are found in the cambium, present the least re- 

 sistance, as shown by various observations on lightning discharges. The 

 minute burned channel, caused by comparatively insignificant lightning 

 discharges, follows down the cambium, indicating that this is the line 

 of least resistance. Moreover, by driving electrodes into a tree to dif- 

 ferent depths and measuring the resistance it can be shown that the 

 least resistance occurs in the region of the cambium. 



The electrical resistance, however, may average throughout the year 

 25,000 ohms more or less in 10 feet of the trunk of a large maple tree. 

 This constitutes a comparatively high resistance. The resistance of the 

 sapwood is very much greater, and probably that of the heartAvood 

 is even higher than that of the sajDwood. 



In determining the electrical resistance it is necessaiy to know the 

 path or course of the current, and the only manner in which the resist- 

 ance of different tissues can be determined accurately is by isolating 

 the tissues. By girdling a tree and scraping the trunk down to the 

 solid wood we can get the resistance of the wood. Mr. G. H. Chapman 

 found the resistance of a freshly cut rock maple stem, li,4 inches in 

 diameter, to be 70,000 ohms with the bark on, but 150,000 ohms when 

 the bark was removed. The electrodes were 1 foot apart. Some of our 

 experiments indicate that next to the cambium the phloem has the least 

 resistance, followed by the sapwood. The outer bark appears to oft'er 

 the most resistance, but when wet the resistance may be somewhat 

 decreased owing to the less resistant film of moisture on the bark. The 

 resistance obtained from an elm tree in summer, with the electrodes 

 10 feet apart and in contact with the cambium, Avas 10,698 ohms, 

 whereas when the electrodes were inserted into the middle of the cortex 

 or phloem we obtained 11,300 ohms resistance. When driven I4 inch 

 into the wood the resistance Avas 98,700 ohms. The outer bark gave 

 198,800 ohms resistance, but when the electrodes were inserted slightly 



