238 MASS. EXPERIMENT STATION BULLETIN 170. 



currents occurs in this way, since the high electrical resistance charac- 

 teristic 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 bj^ direct 

 currents used in operating electric railroads. Attention was first called 



to these cases in Bulletin No. 91, issued 

 by this station, and since the publica- 

 tion of this bulletin other cases have 

 been observed. In all of these cases 

 the escaping current had burned and 

 girdled the trunks for a distance of 

 from 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 obsers^ed in 

 ordinary cases of grounding. In fact, 

 the difference between the burning from 

 direct currents in these cases and that 

 from ordinary cases of electrical injury 

 may be seen at a glance. On electric 

 railroad systems the so-called positive 

 current generally traverses the over- 

 head feed wire, where the injury (burn- 

 ing) takes place. It does not differ 

 materially from that produced by low- 

 tension alternating current wires. In 

 all cases of death from direct-current 

 electricity that have come to our 

 notice, however, the rail was positive 

 and the overhead wire was negative, 

 constituting what is called a "reversed 

 polarity." How common this practice 

 is we cannot say, but apparently it has 

 been employed intentionally at times to 

 prevent electrolysis as well as uninten- 

 tionall}^ by various companies, and is 

 responsible in a few instances for the 

 death of shade trees near electric rail- 

 roads. There is much greater oppor- 

 tunity for extensive burning in the case 

 of reversed polarity than in the regular 

 systems employed. The moisture 

 conditions of the soil and bark are such as to reduce the resistance, 

 and in consequence the film of water and the water-soaked bark 



Fig. 99. - Showing elm tree kilieii by 

 direct current (reversed polarity) 

 from electric railway system. Note 

 eifects of burning at tbe base of 

 the tree. 



