can be approximately gauged from other working undertakings; assume it to be 31%, and the average current will then be 50A. Take the resistance per mile of any wire of about the size required, say 0.25 O per mile; then for a mile of line (lead and return) the resistance is 0.5 0. The power lost in this would (on the average) be (502 × 0.5 / 1 000) or 1.25kW, equivalent to 11 000kWh per annum per mile of line. From the rough estimates the probable output of the station in units and the total annual cost of the plant [interest, depreciation, fuel (if any), and establishment] will be known, giving the cost of each unit generated. In this case take the cost per unit as

0.66d., a fair figure for water power on the assumed conditions. Then the cost of the 11 000 units lost in transmission per mile would be £30, 5s. Now lay out a diagram (fig. 82) with resistance (inversely proportional to cross-sectional area) of copper horizontally and cost vertically, and mark the point where £30, 5s. and 0.25 O per mile meet. Draw a straight line through this from the origin. Next take any three sizes of wire somewhere near the mark and find the approximate cost of a mile run, i.e. in this case 2 miles of wire. Taking this at £75 a ton, or say 8d. a pound, we get the following:

:

Taking 10% of these costs for the charges on the line, mark off the points where these annual costs correspond with the ohms per mile of the wire, thus :—

7/0 S.W.G.

Ohms per mile=0.22. Annual cost £26, 12s.

Draw the curve through these three points, and where this curve is intersected by the straight line the combined costs will be a minimum. The resistance per mile corresponding to this point, viz. 0.22 O per mile, will give the most economical size of wire to use. In the case of 3-phase lines, the same data may be more easily obtained and plotted for a single conductor than for the three wires. In the case of overhead lines results near enough for practical purposes will be obtained by considering the annual charges on the copper alone, though strictly speaking the cost should be increased for large sizes owing to the greater cost of the poles.

In the case of insulated cables the total cost should be used as a basis for the calculation, instead of the cost of the copper alone.

At 20kV working pressure, the commercial considerations entailing the selection of such a voltage also determine to some extent the size of cable most commercially economical from the transmission and distribution point of view. In the case of distribution to a number of areas in various stages of development, the most economical section will be 0.1 to 0.15 sq. in., at 20kV. Where a large amount of power has to be transmitted from a generating station to a main distributing centre, and the cable will be more or less fully loaded, it may be best to use, say, a 0.25 sq. in. cable, working at 33kV. The considerations then to be taken into account are: (a) the limit of current density fixed by working temperature and largely dependent on method of laying and number of cables in proximity to one another; (b) the broad aspect of the whole scheme in which cable considerations are simply an item. (C. J. Beaver.)

It is evident that where water power is in question the generating cost per unit will usually be lower than with steam; if, however, the available power is limited, and can all be sold, every unit lost in the line is a direct loss of revenue to the undertaking at the actual rate of sale, and the average selling price per unit should then be taken instead of the prime cost.

488. Telephones. In every electric supply undertaking, and especially where there is transmission of power over a distance, telephone communication is necessary between the different substations and the power house. For this service overhead lines are

almost invariably used. It is preferable to carry these circuits on a different alignment to the power wires, because when there is a fault in the transmission service interference is likely to result between it and the telephone circuits, and it is at such times that clear speech is most important. In any case a double metallic circuit should be used, and not a single line with an earth return. The wires should be systematically transposed at frequent intervals, so as to neutralise the effects of induction due to variations in the current in the power line and also to the normal cyclic variation in an alternating current line; each 3-phase power circuit should also have two points, approximately dividing the total length evenly, at which the wires are altered in position 120°, so as to give one complete transposition in its length. In long lines several complete rotations or transpositions are allowed. It is necessary to safeguard the line against accidental contacts with the power circuit, whether due to breakage or to trees, and also to safeguard the operators against shocks from the line becoming charged inductively. In some cases it has been necessary, in order to prevent theft of wire, deliberately to charge the line to high pressure at night.

Although in very long lines the size of the telephone conductor becomes important, it is not so within the limits of transmission of power. Copper, phosphor bronze, mangan bronze, and silicon bronze wires are mostly used, and a wire of 100 lbs. per mile (say No. 14 S.W.G.) is large enough electrically; a smaller size would be too liable to mechanical damage. For long spans copper wire is unsuitable, and these special bronzes have much greater tensile strength; but as the alloy and the strength are increased the conductance is decreased in a much greater proportion (see Table 98, § 483). The weight of a telephone wire may be taken without serious error from Table 17, § 181, according to its gauge or sectional area, whether it is copper or bronze. The cost of mangan bronze is normally about 1s. 3d. to ls. 6d. per lb., but varies with that of copper; phosphor bronze and silicon bronze are about 20% dearer.

Ordinary receivers cost from 8s. to 15s. each; transmitters from 10s. to 21s. or more. Complete magneto instruments (Hunningcone and the like) cost from £2, 10s. to £4 per set, or for long-distance working up to £7. A switchboard for 10 lines, including operators' instrument, costs about £16.

A useful paper on telephone troubles in the tropics was recently read before the Institution of Electrical Engineers by W. L. Preece (Journal I.E.E., 53, 545). Amongst other points specially worthy

of notice in India are the following:-It is not uncommon for insects to find their way into a subscriber's instrument through the switchhook; this should therefore carry a brass plate which keeps the slot in which the arm works entirely covered. There should be no terminals above the instrument, but the conductors should be taken through holes into the case and sealed up. Internal wires should be separated as much as possible. Lightning protectors should be fixed where the wires enter the building, and not by the instrument; otherwise a fire may easily result. For the overhead line the use of glass insulators of the oil-filled type is suggested tentatively, as a protection against insects. As a protection from lightning troubles on the line, causing breakdowns at the pole box, the use of the vacuum type of protector is recommended, in which two carbon blocks are inserted in an exhausted glass tube, the opposite surfaces being serrated and fixed about in. apart. A fuse in the circuit in addition is always essential. The use of an additional earthed iron wire, on the top of the poles, is also recommended. (See also § 484.)

CHAPTER 24.

Specifications: Depreciation and Maintenance.

Specifications and Allied Matters.

489. I.E.E. Model Form of Contract; Specifications Generally.In most cases the "Form of Model General Conditions recommended for use in connection with Contracts for Electrical Works will be found a useful guide in connection with any large projected works. This document was originally prepared in 1902-03, and recommended for use by the I.E.E. It consists of Form of Tender, Form of Agreement, Form of Guarantee, and General Conditions relating to everything connected with such works. It has recently been revised (1913) and issued by the Institution. Some of the details may not be altogether applicable where work in tropical countries is concerned, but the model form' will be more satisfactory on the whole than any local production not drawn up by a competent consultant. In addi tion to the above, a specification and drawings and (in the case of work paid for by measurement) a schedule of prices are necessary.

6

It is notorious that, in a large number of cases, the specification of works to be carried out abroad, and the preliminary investigation leading thereto, is left to the firm of engineers who have originated, and are predestined to carry out, the work. The engineer nominally responsible for the proposed work, often has little or no share in its initial stages. So long as the firm employed are told that others will be invited to tender to their scheme, and are paid for their preliminary work, this is unobjectionable. It is, however, clearly unfair to employ a firm in this way under any other conditions, unless they are certain to carry out the work also.

In succeeding paragraphs brief notes are set down relating to the data required to be given or received in regard to items of plant, etc. So far as the internal wiring of buildings is concerned, a speci