THE SIZE OF THE MAINS.

The area of the conductor depends greatly on the distance to be traversed before the electricity is used, the object being not to allow the resistance of the exterior circuit to increase beyond a certain extent, which is fixed for each particular system.

If too small a conductor is used, a considerable amount of power is wasted in heating it; if too large, an outlay for copper is sunk: so that a mean has to be struck, which has the object of making the loss as low as possible.

Let us suppose a generating station has been erected containing motive power equal to 1500 horse-power, allowing each incandescent lamp to absorb one-eighth of a horse-power, which is considerably more than is actually the case; this generative force will enable us to work 12,000 lamps, say of 20 candle-power each. The ordinary Swan lamps require 1·4 ampères of electric current, so the total amount to be taken by the mains will be 16,800 ampères. The following table gives the weight and resistance per 100

yards and per mile, also the current in ampères which can be safely carried per 1000 ampères per square inch of section of pure copper wire. As commercial copper is seldom higher than 96 per cent. conductivity, allowance must be made in calculating the resistance. The rule in the provisional order as to the maximum current which can be safely taken by any pure copper wire allows of a greater amount of current to be passed through, providing the current does not exceed 10 ampères.

The following is a useful formula, which, however, gives a rather higher amount than 2000 ampères per square inch of section, which is the maximum allowed in the Orders:

Where C current safely carried by copper wire of 96 per cent. conductivity; W = weight of the copper wire = 0.32 lb. per cubic inch; R = resistance.

A single strand of No. 3 copper 259 diameter wire one mile long has a resistance of 8566 ohms, and the current carried is 100 ampères. Although a cable made of 19 such strands could be made, it would be far more convenient to have three cables, each composed of seven strands, making 21 in all, which would safely carry a current of 21000 ampères. To comply with the Order twice this number of cables would be required.

The resistance of each of these cables would not be more than 1222 ohms per mile ;* the first cost would be considerable, but it must be remembered that, unlike the iron pipes of gas and water, the value of copper is little depreciated by use, so that in the event of a larger main being substituted, or the project abandoned the break-up value would not be very far short of the original cost. The price of copper varies with the price of Chili bars, the current quotations for which are published daily. It may be said to average 887. per ton, which brings the cost of No. 3 gauge wire to about 477. per mile.

MOTORS AT THE GENERATING STATION.

In many cases, no water power will be available, and it will be necessary to erect steam-engines and boilers to act as motors for the electric machines.

*The Ohm is the unit of Resistance.

Where sufficient space can be obtained, the style of engine best adapted for a permanent installation is the horizontal compound condensing engine, of which there are several varieties. It will be sufficient for the objects of this paper to state that, as an electric light engine has to work for a considerable time without stopping, it must have ample bearing surfaces and efficient means of lubricating same. Either surface or jet condensers may be used; if the former, in some cases it might be possible to make use of the water supply of the town for the purpose of condensing.

Engines of this description are specified by the makers to give an indicated horse-power under 2 lb. of coal consumed, in fact it is quite possible to obtain an indicated horse-power for 11⁄2 lb. using 14 to 15 lb. of feed water per hour; a certain reserve of power is necessary, so that if for an installation of 20,000 lights, six engines, each of 325 indicated horse-power, were used, a seventh should be kept in reserve. The whole of these engines would be arranged so as to drive on to a line of counter-shafting in such a manner that any one could be stopped without affecting the others.

The usual plan is to transmit the power by means of either belting or rope gearing; in both cases allowance must be made for the wear of these, which cannot be worked when there is any chance of their breaking.

Where first cost is not a special object, a spur wheel gearing into a pinion with wooden teeth would probably be cheaper in the end. In the installations on the Edison system, high-pressure engines coupled direct to the dynamo are used, but although great economy in space is secured, the chances of a breakdown are increased.

BOILERS.

Compound condensing engines give the best results with boilers of the Lancashire or Elephant type, but if steam is to be got up quickly, locomotive boilers may be used, or the water-tube boilers, -of which there are numerous types-more or less similar to those employed at the Edison installations. A spare boiler should always be erected for every six working, and each of these should be of such dimensions that they can keep steam easily without forcing, so that in the event of one of the boilers being disabled the others will be able to make sufficient steam. The extra boiler is for the purpose of being substituted for any of the others it is necessary to clean.

The approximate cost of an installation of 2000 indicated horsepower would be

Six 100 horse-power nominal compound condensing
engines, erected at 16001. each

One in reserve

£

9,600

1,600

If a surface condenser be used, an extra sum must be added for the metal tubes.

DYNAMOS.

Although the electric machines would not be erected under the proposed scheme at the expense of the authorities, some provision must be made for them.

A good plan is to fix strong battens into a concrete foundation in such a manner that two battens are bolted together so as to leave a space through which a holding-down bolt can slide. Each dynamo will rest on two iron transverse pieces resting on these battens, and arranged so as to give lateral movement, while the dynamo and transverse pieces can be fixed in any position on the battens to allow for the tightening of the belts.

Horizontal machines have, as a rule, their centre of gravity near the ground, so do not require any holding-down bolts, but those with the magnets in a vertical direction must be firmly secured.

The lowest cost of 1000-light dynamos may be taken at 7507. for 1200 lamps, but as soon as a demand is established a very considerable reduction may be expected.

It is hardly necessary to point out the absolute necessity of having an efficient governor on the engine, so that its speed may not be altered by the work it is called on to perform.

For large engines there are several well-known types of efficient governors; they are all, however, more or less complicated, and the author will pass by them to describe an hydraulic governor, specially devised for electric light engines, which has the merit of being cheap and simple.

It consists of a small centrifugal pump driven by gearing from the main shaft of each engine, arranged so as to deliver a column of water against a weighted piston which is directly attached either to the throttle or expansion valve of the engine.

As the quantity of water delivered varies in proportion to the speed, a very delicate regulation may be maintained by more or less closing the mouth of the delivery pipe. The water is discharged into a small cistern, whence it is led back to the suction of the pump. This governor is being largely used in France for electric light installations, and with very satisfactory results.

SECONDARY STORAGE BATTERIES.

These batteries-often wrongly termed accumulators—act as magazines of electricity which can be drawn on when required; they are destined to play a very important part in the electric lighting of the future.

Although the success of the Edison system in London and New York, where no batteries are used, is often quoted as a reason why their use is unnecessary, it is impossible to economically maintain a constant supply of electricity without their use. Although various forms of batteries have been shown at work, all have failed when put into practical requisition long before the time given by the inventors.

The objection made to the use of secondary batteries is that, first, they do not last, but have to be frequently renewed, and, secondly, that they cost too much.

The first complaint is one which still creates a difficulty to their adoption, one of the causes of which is by local action taking place on the oxygen or positive plates, which causes the peroxide of lead already formed on the plate to be reduced to a lower state of oxidation, and the lead backing supporting it to be gradually oxidised and eaten away. The second drawback, that of cost, is already partly remedied, and will be more so when batteries are put in the market at a price per cwt. unburdened with patents of very questionable value. Minor details, such as suitable connections and jars that