1919.] Anderson.-—Long-distance Electric Transmission-lines. 
95 
THE LOCATION OF LONG-DISTANCE ELECTRIC 
TRANSMISSION - LINES. 
By G. P. Anderson, B.E. 
In connection with the Government’s intention of tapping some of the 
sources of water-power in this country, one of the principal, if not the 
biggest, undertakings will be the erection of transmission-lines to distribute 
the power to the various centres and also to country districts. Nearly 
four hundred miles of these main distribution-lines have already been 
located in the North Island alone, and there is vastly more to be done before 
the country is completely reticulated. 
This paper is written with the object of describing the methods which 
have been found necessary in the field and in the office in order to expedite 
the work of surveying and of plotting the results, and the precaution to be 
observed in locating the supports according to the configuration of the 
country. The paper has further for its objects the preparation of charts 
suitable for application to field-work, and explains the principles and 
method of constructing the charts. 
The location of long-distance transmission-lines may be regarded as 
determined in three stages :— 
(1.) The route is usually settled beforehand from maps and considerations 
of market. It may have to be modified to suit the natural features of the 
country, which may be rough, swampy, or subject to fogs or salt-spray or 
some other undesirable feature. 
(2.) General location along a valley or a ridge or across a flat can usually 
be decided after a casual inspection of the country. It may be necessary, 
however, to run trial lines, and to take into account the cost of construction 
and facility of patrol, as well as length and mechanical and electrical 
features, before making a final choice. 
(3.) Precise location entails the exact fixing of lines and angles and the 
spacing of poles so as to give the requisite clearance and ensure stability 
under all conditions. It is to facilitate this that these notes and diagrams 
have been prepared. 
We have four primary factors to consider—viz., tension in the wire, 
weight of wire per unit of length, total loading, and temperature ; and four 
secondary factors—viz., sag-deflection, sag-extension (difference in length 
between arc and chord), elastic extension, and thermal expansion. 
The curve assumed by a flexible wire between two points of support 
is the catenary. It will be shown later how nearly the catenary approaches 
in form to the parabola when the value of the tension is great compared to 
the weight per unit length. The catenary is produced as the result of 
uniform loading along the arc, while the parabola is obtained by loading 
uniformly along the chord. 
Fig. 1 represents a catenary suspended at the points A and B. Consider 
a length OP of the curve. This will be in equilibrium under the action of a 
tension t at P acting along the tangent to the curve at P, a tension T 
acting along a tangent at 0, which is horizontal, and a force representing 
the weight of the length of OP acting vertically through the centre of 
gravity of the section at G, the value of which is ws where o) is the weight 
