MASS. EXPERIMENT STATION BULLETIN 156. 



for leakage, the current traversing the film of water on the tree to 

 the ground. The result of contact of a wire with a limb under these 

 conditions is a grounding of the current and burning of the limb due 

 to " arcing." The vital layer and wood become injured at the point 

 of contact, resulting in an ugly scar and sometimes the destruction of 



M 



MX 



•ACCELERATION 



H -H 



-RETARDATiON- -DEATH- 



FiG. 3. — Diagram showing range of electric current affecting 

 plants. M = minimum; O rr optimum, or current producing 

 greatest stimulus; MX= maximum, or deatli cuiTeut; R to 

 MX = retardation current. 



the limb or leader. In a large number of tests made by the aid of 

 sensitive instruments Avith guy wire and other connections of wires to 

 trees we have never found any leakage during fair weather, or when 

 the surface of the tree is dry. Since the amount of current that can 

 be passed through a tree depends upon the resistance and electro- 

 motive force, we shall consider this resistance at some length. 



Electrical Resistance of Trees. 

 The electrical resistance shown by trees is quite great, otherwise more 

 injui'y might result from contact with live wires. The following table 

 (I.) gives the electrical resistance of 10 feet of a maple and elm, each 

 tree being about 2 feet in diameter and the electrodes 10 feet apart. 

 These resistances were determined by a Western Electric Company com- 

 bination bridge rheostat and galvanometer, and a large battery. Other 

 resistances, however, have been obtained by means of the electro-motive 

 force, and a known current passed through the tree, the two methods 

 agreeing in their results quite closely. The table, which is taken from 

 one of our previous publications,^ is one of many. 



Table I. — Showing Average Electrical Resistance {in Ohms) of Maple 

 {Acer saccharum Marsh) and Elm {Ulmus americana L.) cov- 

 ering a Period of nearly Three Months, Resistances taken on the 

 North, South, East and West Sides of the Trees about Midday. 

 Electrodes 10 Feet Apart. 



Tkee. 



Month. 



North. 



Maple, 



Elm. 



April, 

 May, 



June. 



April. 



May, 



June, 



20,800 

 24,500 

 22,550 

 24.275 

 17,375 

 17.883 



I Electrical ResLgtance of Trees, G. E. Stone and G. H. Chapman, 24th Ann. Rept. Mass. 

 Agr. Exp. Station, 1912, Pt. I., p. 144. 



