276 ELEMENTS OF ELECTRICAL ENGINEERING. 



between them, depends in part upon the size and shape of the 

 wires as explained in Art. 121. 



Whenever, in a given case, any one of these conditions de- 

 mands a larger wire than would be required by any of the other 

 conditions, the larger wire should be used. Frequently an engi- 

 neer is guided by one only of the above conditions in laying, out 

 the preliminary plans for a distributing system. When this is 

 the case the preliminary plans should be examined carefully to 

 see that all of the conditions are satisfied before the plans are 

 finally adopted. 



115. Mechanical stresses in aerial wires and their supports.* 

 Stresses in the supports. The stresses in the insulator pins, 

 cross-arms, and poles are : (a) The stresses due to the weight of 

 the wire plus the weight of an occasional coating of ice ; this 

 weight is to be considered as resting directly upon the insulators 

 and constituting a force acting vertically downwards, f () The 

 stresses due to the unbalanced tensions J of the wire on the 

 opposite sides of an insulator. The tensions of the wire on the 

 opposite sides of an insulator are in nearly eveiy case sensibly 

 equal in value and unbalancing occurs only where the wire termi- 

 nates or changes its direction. In the case of a straight pole-line 

 on a slope the tension of the wire is generally greater on the 

 down-hill side of the pole, but the unbalanced force is in this case 

 a force acting vertically downwards, that is, a given insulator 

 supports a large part of the weight of the lower span of wire and 

 a correspondingly small part of the weight of the upper span of 

 wire, (c) Stresses due to wind pressure. 



( i ) The weight of wire and ice produces, in the poles and pins, stresses of simple 

 compression, which stresses may nearly always be neglected, inasmuch as poles and 



* A discussion of the details of pole-line construction is beyond the scope of this 

 text. Information concerning these details may be found in " Electrical Transmission 

 of Energy," A. V. Abbott, 1905 edition, Chapter III. 



f This force is the sum of the vertical components of the tension of the wire on the 

 two sides of an insulator. 



J Horizontal components of the tensions, inasmuch as vertical components are con- 

 sidered under tf. 



