at the bottom of the trough, and short circuits will be come very probable. The second reason why an increase of temperature is objectionable is, that the resistance o the cable becomes greater, causing an additional waste of energy. The third reason is that an undue increase of temperature may in an overhead line actually cause a fire risk at the points where the cables are attached to buildings, and may in an underground line become very unpleasant to foot passengers.

It becomes thus a matter of great importance to determine beforehand what rise in temperature is to be expected in each given case, and if that rise should be found to be greater than appears safe, provision must be made to increase the rate at which heat is carried off. This can generally be done by increasing the superficial area of the conductor. Say we have one circular conductor of 1 square inch area, and find that with 1,000 amperes flowing it would become too hot. Now by splitting up this conductor into 10 separate wires each one-tenth of a square inch cross-sectional area we have not altered the total amount of energy transformed into heat, but we have increased the surface exposed to the cooling action of the surrounding air in the ratio of 1: √10, and therefere the ten thin wires can dissipate more than three times the heat, as compared with the single thick wire. Professor Forbes gives a table-reproduced on page 191from which it can be seen at a glance what current a wire can carry when the rise in temperature is 9° and 26° above that of the surrounding air.

Legal volts 96 per cent. conductivity copper. Heating of bare copper wire,

emissivity 00025 C.G.S. units.

CHAPTER VIII.

Circuits for Electric Transmission-Circuits for Electric DistributionRelative Importance of Insulation-Aërial Lines-Insulators-Attachment of Conductor to Insulator-Joints-Couplings-Material for Aërial Lines-Estimate for Aerial Line-Protection from Lightning-Underground Lines-Edison Mains-The Three-Wire System-Various Systems of Underground Conduits-Lead-Covered Cables.

THE question whether the line should be carried overhead or be placed underground, depends on a number of local circumstances, but as a rule it will be more economical and sufficiently safe to use aërial conductors for the transmission proper of energy, whereas for its distribution underground cables are preferable, and in some cases indispensable. The time is fast approaching, and in America may be said to have already arrived, when no further addition to the vast network of overhead telegraph and telephone wires in our towns will be permitted, and it is quite certain that no exception in favour of wires containing, so to speak, a large store of potential energy, will be made. Electric Light and Power Companies have realized this state of affairs from the beginning, and where they have come forward with definite proposals for a general supply, they have always arranged for their distributing plant to be placed underground. The case is different when electric transmission over a long distance, and possibly across country, is involved. Here the danger from breakage of an overhead wire can be almost entirely avoided by placing the supports at frequent intervals a precaution not always possible in towns where the width of streets and places often necessitates an ex

town.

cessively long span from one support to the other—and if a wire should break, the chances of anybody being hurt are infinitely smaller than in the crowded streets of a We have already seen that power can only be economically transmitted over a long distance by the employment of a high electro-motive force, and hence the proper insulation of the line becomes a matter of the utmost importance. If, in a town district supplied with current at, say, 100 volts, a small leak of a few amperes should take place—and Mr. Edison's experience in the New York Central Station seems to show that such leaks do occasionally occur-the loss of energy, as compared to the thousands of amperes sent out from the station, is very trifling, but if an equal leak should be developed in a circuit of two or three thousand volts, it might very easily absorb all the current which the generating dynamo can pour into the cables, and no energy at all could be obtained from the motor. In an overhead line faults of insulation are not so easily developed, and if they occur, are more easily discovered and repaired than in any of the underground systems, which as yet have hardly had any prolonged trial to show their practical value; and for this reason it will be safe to assume that aërial conductors will be almost generally used for the transmission of electric energy at high potential over long distances, and that underground conductors will generally be used for the distribution of electric energy at low potentials.

Aerial Lines.

The conductor is generally a naked wire or cable of copper, iron, phosphor bronze, or silicon bronze, but slightly insulated conductors are sometimes used. The

insulation gives some protection against short circuits, which might otherwise be caused by other wires, branches of trees, or other bodies falling across the leads, and it has also the advantage of increasing the cooling surface

of the conductor, thus reducing the temperature. At first sight it might seem surprising that a wire coated with insulating material, which is necessarily also a bad conductor of heat, should become less heated than a naked wire. But such is the fact ascertained by experiments, and explained on the ground that quiescent air is the