the plant being conveyed directly to the transformers without going through the main switchboard. This enables the whole of the high

from a small day load plant at the generating station, the current from

FIG. 180.-Diagram of L.T. distributors (Hastings).

tension system to be shut down during the hours of light load, thus entirely preventing the usual heavy losses in magnetising current and cable-charging current; and, what is, perhaps, of even greater importance, the necessity of working on live high-tension connectors is entirely avoided. It may be thought that the drop of pressure when feeding through the distributors alone would be prohibitive. As a matter of fact, the day load during the long summer days is less than 10 per cent. of the maximum night load, and consequently the distributors will transmit the current ten times the distance with the same drop of pressure.

To switch off the H.T. system, the main L.T. switches in each sub-station are first opened, thus disconnecting the secondaries of the transformers from the L.T. network. The sub-station at the works is then changed over to the day load plant, and the whole of the H.T. system is shut down. To change back again, the H.T. cables and transformers are run up to full pressure and the L.T. sub-station switches are closed. These switches may be closed through pilot wires run from each sub-station to the generating station. A pilot wire board at the works is equipped with small switches for operating the main sub-station switches, with lamps on each pilot wire which are extinguished should a transformer break down and cause its discriminating cutout to operate, and with a static voltmeter by means of which the distributing pressure at any sub-station may be ascertained.

Each sub-station is connected by a private telephone line to the works and to other sub-stations.

CHAPTER X.

LONG DISTANCE TRANSMISSION.

Determination of line pressure-The use of copper, aluminium, or steel for overhead conductors-Wooden or steel posts for transmission lines-Insulators, glass and porcelain-Leading in wires-Cable charging devices-Pressure rises due to open air arcs-Lightning arrestors: Thomson,' 'Siemens,' 'Wurtz,' and 'Stanley'Arrangement of choking coils and lightning arrestors- Requirements that should be fulfilled by lightning arrestors-Earthed guard wire for lightning protectionRegulation of pressure, 'Cowan-Still' regulating transformer- Paderno' three-phase transmission scheme-Thury's' E.H.T. constant current system; simplicity of controlling arrangements; regulation of motors; excess potential cutoutValtellina Electric Railway; motors coupled in cascade.

If there is one field, more than any other, in which electricity stands unrivalled, it is in the transmission of energy over long distances. The commercial transmission of large powers over lines from 100 to 300 miles in length is now a matter of daily occurrence, both in Europe and America, particularly on the West Pacific coast; one of the most notable instances being the Standard Bay Counties line, carried out by the Stanley Electric Manufacturing Co., under the supervision of Dr Perrine.

In this country the demand for such schemes has not, so far, arisen, and is not likely ever to do so; it is, in fact, probable that the use of long distance transmission lines will here be confined to electric railway work, though lines of moderate length will doubtless be largely used by some of the power distributing companies.

Long distance transmission, to be a commercial success, entails the use of overhead wires, and the working at very high pressures. Both these factors introduce problems in connection with the controlling arrangements that do not arise, at least to the same extent, when working underground cables at moderate pressures.

Determination of Line Pressure.-One of the first questions to be determined upon is the line pressure. A rough and ready rule suggested by Mr C. F. Scott, for determining the most economical pressure, is that the pressure in thousands of volts should equal one-third of the number of miles over which energy is to be transmitted.

Conductors. Overhead conductors are usually of copper, though aluminium has been employed in several transmission schemes. The latter has a higher tensile strength than copper, compared to its specific gravity,

but its conductivity per square inch section is lower, and consequently the surface exposed to wind pressure is considerably greater.

Dr Perrine in his book on Conductors for Electrical Distribution gives the

following relative values of copper and aluminium for a given length and

For crossing large rivers where there are no bridges cast steel wire is used, on account of its high tensile strength. In these cases the high resistance of this wire is not an important factor, considering the short

lengths employed. Such spans have been erected in Egypt and India exceeding a mile in length.

Posts. The posts for supporting the overhead lines may be wooden or steel structures. Both are largely used. Fig. 181, reproduced from a photograph of the transmission line between Paderno and Milan, is an example of the latter construction. The two lines of posts carry between them six parallel three-phase lines-eighteen 9-mm. wires in all. The distance between the supports is about 200 feet, and the total length of line is about 20 miles, the pressure of the supply being 13,500 volts.

Fig. 182 shows the wooden posts of the Hudson River power