18 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 
portion. In ashort time the cell formation returns to normal. Ulti- 
mately, as the growth of the tree proceeds, these lightning rings, 
always following one definite annual ring, are deep within the wood, 
extending completely or partially around the circumference over a 
varying distance. When the tree is worked up for lumber certain of 
the boards may have such lightning rings extending completely 
through, both in width and length. Such a board then consists of 
two layers of wood held together by a zone of abnormal structure 
forming a plane of cleavage. Checking often occurs along this line, 
since the continuity of the medullary rays may be interrupted. Such 
checks are striking, since they invariably are tangential, following 
an annual ring on end section or radial face but not visible on the 
tangential face. This 
Is not at all an un- 
common defect in air- 
plane timber. An ab- 
normal number of 
resinducts may be 
found in the annual 
ring following many 
types of mechanical 
injury, but for prac- 
tical purposes there is 
no difference between 
such so-called trau- 
matic resin ducts and 
the abnormal ducts 
formed as a result of 
hghtning injury. 
It isself-evident 
that wood with these 
hghtning rings must 
be used with discre- 
tion. Eventhough 
the lightning ring 
does not check on dry- 
ing, when a mem- 
Fig. 4.—Section from a finished interplane strut, showing ber with this defect is 
a small lightning injury in Sitka spruce. p ut under severe 
strain and stress a 
serious check may develop. Of course, every member showing a 
hghtning ring need not be considered valueless. Such a defect in 
the stream line of a strut, for example, would be trifling, while a 
much shorter ring in the butt or inner bay of a wing beam, particu- 
larly if in the same plane as the bolts, would be serious. The same 
ring in the tip of such a beam could be overlooked. 
The detection of lightning rings in rough lumber is exceedingly 
difficult, unless accompanied by small wounds, which is sometimes 
the case. Then such wounds must be scrutinized closely for the 
presence of a lightning ring. Two or more of these wounds, which 
resemble sapsucker wounds, occurring on the same annual ring 
and connected by a lightning ring, are sometimes found. Figure 4 
shows one of these wounds on an interplane strut, in this case not of | 
a Te ys oe eee 
. 
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