72 Diagnosing of Diseases 
bole there may not be any more consequences than accom- 
pany any ordinary or frost-split wound which may be healed 
by callusing, but when it follows a spiral course, killing 
cambium on all sides of the tree, the result is like that of 
girdling. 
Very lately it has been discovered and experimentally 
demonstrated ' that electric shocks of low tension occurring 
during thunder-storms, especially in winter, are the frequent 
cause of the phenomenon described above as siaghead, the 
leader and upper portions of the crown for from five to ten 
feet or more being killed while the rest of the tree remains 
uninjured. 
Every tree owner should look out for the wire stringer, 
who not only disfigures the tree by chopping off branches, 
regardless of consequences, but introduces the danger of 
electrocution. Trees and branches, beating against elec- 
tric wires in winter storms, wear off the insulation and thus 
establish short circuits, which under certain weather condi- 
tions as, for example, in a thunder-storm, may bring about 
the total destruction of a long-cherished old tree. 
According to the latest investigations ? there is a difference 
in the effects of direct currents which are used chiefly in 
operating electric railroads, and of alternating currents of 
the electric light plants, which, although carrying a higher 
current, appear to be less disastrous. 
In moist or wet weather, when the tree is covered with a 
film of water, the current causes, at the point of contact, an 
injury to the tree, which provides favorable conditions for 
leakage through the film of water, grounding the current and 
burning the limb, partially or entirely killing the cambium 
1See Forestry Quarterly, Feb , 1904 
2“Tnjuries to Shade trees from Electricity.” Bulletin No. 91, Mass. 
Agr. College. 
