28 METHODS OF COMMUNICATION FOR FOREST PROTECTION 



height above the ground; and serving as supports for a special two-piece porcelain 

 insulator through which the lime wire passes without being rigidly attached. Tree 

 lines are never straight as are pole lines. On curves the line wire is always placed 011 

 the concave side and there is a very much greater allowance for sag than that required 

 for normal contraction of the wire. Supports on tree lines are generally about 65 

 per cent greater in number than on pole lines, and sag allowance is from four to six 

 times as great. 



Section 34 Construction Methods Contrasted 



Pole lines are built only in open country and along cleared roads or, if they 

 must be carried through timber, a sufficient right of way is cleared to ensure that 

 trees will not fall across the line. They are built rigidly and the wires are expected 

 to stay up unless the poles themselves give way. 



Tree lines are designed to be built only through standing timber, where no 

 attempt is made to clear a sufficient right of way to ensure against falling timber. 

 Instead, special construction is used for the purpose of ensuring that when trees fall 

 across the line, the wire will be carried to the ground without breaking or, in case 

 a number of trees fall in a short distance, the tie wires will break and a considerable 

 portion of the line wire be brought to the ground still unbroken. 



A further danger that tree-line construction overcomes arises from the swaying 

 of the supporting trees in the wind. This does not occur in pole lines but is quite 

 obviously a matter of some importance in tree lines. In short, tree lines are designed 

 to give way and- fall to the ground when subjected to unusual strain ~by falling timber 

 but any breakage that occurs is designed to come in the tie wires and not in the line* 

 wire. The underlying principle is that a broken line wire means a dead circuit but 

 a line wire of a grounded circuit may still function as a telephone circuit when a 

 large part of it is on the ground, or even when buried under dry snow. If the ground 

 is damp it will function very imperfectly and if it lies in a stream or pool of water 

 it is no better than a broken wire, but a considerable portion of a grounded circuit 

 may lie on ordinary dry or frozen ground without entirely destroying the usefulness 

 of the line for telephone purposes. 



Tree lines, however, have certain very definite limitations. It is impracticable to 

 use copper wire with this type of construction. This limits the length of tree lines to 

 .about 125 to 150 miles using No. 9 iron wire. If it is necessary to build a metallic 

 circuit to ensure against induction it is not practicable to attach both wires to the 

 same supports, as is done with pole lines, but two separate parallel lines must be built. 

 Tree lines are never as perfectly insulated as are standard pole lines and, therefore, 

 are more subject to losses of current and other difficulties in transmission. This is 

 true no matter how well they are built and is inherent in the methods and line equip- 

 ment employed. 



Section 35 Relative Cost of Different Types 



It is a mistake to assume that a tree line is necessarily cheaper than a pole line 

 even where there are sufficient trees available to, furnish supports for the line. Par- 

 ticularly is this true when maintenance charges and operating efficiency are taken 

 into consideration. 



The only cost elements of pole-line construction that are greater than those of 

 tree-line construction are the cost of the poles set on the line and the cost of right-of- 

 way clearing. The cost of the former will vary with the accessibility and durability 

 of poles, facilities for distribution, and character of the soil as regards cost of digging 

 holes. The cost of the latter depends on the nature of the forest and underbrush. 



On the other hand all other cost elements are likely to be higher for tree lines 

 than for pole lines. It is never advisable to use smaller wire than No. 9 B.W.G. iron 

 wire on a tree line. On a pole line, however, No. 12 B.W.G. iron wire may be used on 



