8 MASS. EXPERIMENT STATION BULLETIN 156. 



95 per cent, gi-eater than that occurring about the negative electrode. 

 In each case it extended about twice as far above and below the point 

 of contact as out to the sides of the electrodes, thus showing a tendency 

 of the current to spread laterally as well as vertically, but more largelj'" 

 vertically. 



The immediate area around the electrodes was more affected than that 

 further remote. There was an area of tissue about 5 inches long be- 

 tween the large and small oval burning that was uninjured, showing 

 that burning was confined about the electrodes. The current traversing 

 the film of water on the bark between the electrodes was not sufficient 

 to destroy all of these tissues at that jDoint. 



If a milliammeter had been i^laced in the circuit when the tree was 

 w^et a greatly increased current would have been detected, since the cur- 

 rent in this case traversed the less resistant film of moisture on the 

 bark. But the electrical resistance of the vital laj^er under such con- 

 ditions would remain practically the same as when the tree was dry. 

 The burning and injury in this case resulted from the heating of the 

 film of moisture, which became so intensely heated that the vital tissue 

 was destroyed, especially near the point of insertion of the electrodes. 

 The more the film became heated the greater was the lessening of the 

 resistance and increase of the current. 



Practically all of the burning of trees from either alternating or 

 direct currents occurs in this way, since the high electrical resistance 

 characteristic of trees does not permit injurious currents to pass through 

 their tissues. 



Death of Trees from Direct Current. 

 Instances are known in which large trees have been killed by direct 

 currents used in operating electric railroads. So far as we know atten- 

 tion was first called to these cases in Bulletin No. 91, issued by this 

 station, but since the publication of this bulletin other cases have been 

 obsei-ved in which the escaping current had burned and girdled the 

 trunks for a distance of 5 to 10 feet from the base, the point of contact 

 of the feed wire with the limb 18 or 20 feet above, showing little or 

 none of the characteristic local burning effects usually observed in 

 ordinary cases of grounding. In fact, the difference between the burn- 

 ing from direct currents in these cases and that from ordinary cases 

 of electrical injurj^ may be seen at a glance. On electric railroad sys- 

 tems the so-called positive current almost always traverses the overhead 

 feed wire where the injury (burning) takes place. This differs only 

 slightly from that produced by low-tension alternating current wires. 

 In all cases of death from direct current electricity that have come to 

 our notice the rail was positive, and the overhead feed wire was nega- 

 tive, constituting what is called a "reversed polarity." How common 

 this practice is we cannot say, but api^arently it has been done inten- 



