The instruments usually required on a switchboard are: (1) Watt-hour meters, (2) Ammeters, (3) Voltmeters, (4) Recording Ammeters and Voltmeters.

(1) The Watt-hour meters should be placed on the machine circuit, not on the feeders; they will thus operate chiefly on full load and near about the same rate, and errors due to their indications following an erratic law will thus be practically eliminated. The most suitable are the Aron and the Thomson, described in Chapter XXV. The latter is influenced by strong fields, while it is not desirable to place the former in them. Conductors in the neighbourhood of the meters should be concentric.

(2) Ammeters may either be connected directly in the circuit, or may be connected as a shunt to a low resistance of known value, through which the current to be measured passes. The advantage of this latter course is that the instruments can be grouped together in any convenient position, irrespective of where the conductors are. The disadvantage is that temperature errors cannot be fully allowed for. There are two sources of error due to temperature, viz.: (1) changes in temperature of the coil of the instrument, due to changes in that of the surrounding air; and (2) changes in the resistance of the conductor, across the terminals of which the instrument is connected, due to the heating caused by the current to be measured.

It is possible to eliminate the first error and leave the second uncompensated for, or else to get rid of the second by employing a metal having a negligible temperature coefficient and neglect the first error A compromise has to be effected by partially correcting for both.

The so-called 'edgewise' form, in which the index is bent round at a right angle and moves over part of the surface of a narrow cylinder, the axis of which is the centre of motion, is most convenient, and enables a large number of ammeters with very open scales to be got into an exceedingly small space, while they can be more easily read than the dial form. A set of twelve such instruments is shown in fig. 54.

The essentials for a switchboard ammeter are: fair, though not necessarily minute, accuracy; dead-beatness; strong construction, especially as regards the pivots; an open, coarsely graduated, conspicuous, direct-reading scale; capacity for heavy overloads of short duration; freedom from heating.

(3) Voltmeters for high pressures are almost necessarily of the electrostatic type, while for low pressures the instruments are very reliable, but suffer from contracted scales. Both the magnetic and moving coil types are available. The most desirable is the moving coil.

The essentials for switchboard voltmeters are extreme and constant accuracy over a short range, say, for 10 per cent. on either side of the mean standard pressure; dead-beatness; strong construction; bold pointer, and clearly visible, large graduations; freedom from heating during continuous.

working. The 'illuminated dial' form, in which the pointer moves over a translucent scale lighted from behind, is admirable.

Such an instrument is shown in fig. 55.

(4) Recording instruments are those which show, by means of a line traced on a graduated sheet of paper kept travelling beneath a pen, the mode of variation of current or pressure. All are open to the objection that the friction between the paper and the

pen interferes with the movement of the pen and with the rate of the clock, while the inertia of

the moving parts tends to wipe out small variations and exaggerate large

ones.

One of the most satisfactory record

ing ammeters is that made by Messrs Elliot Bros., in which a continuous strip of paper is unreeled beneath the pen; while one of the best recording voltmeters is the Kelviu, in which the pen has a point resembling that of a stylographic pen.

As an example of a switchboard, designed by the Author, embodying the principles enunciated in this chapter, may be given that shown in fig. 56.

This is of the gridiron type, with bare copper rod connections; it is constructed wholly of incombustible materials-metal and porcelain-with the exception of ebonite bushes and washers to insulate the bolts. The positive

and negative conductors, differing in potential by 400 volts, are on two separate boards and some 8000 H.P. is controlled; the instruments used are of the edgewise type.

FIG. 56.-Switchboards controlling 8000 H. P. embodying principles enunciated.

CHAPTER XVIII.

DISTRIBUTING MAINS DRAWING-IN SYSTEMS.

THIS subject is one that demands the closest attention, for the proportion of the total capital invested in the undertaking represented by the mains may amount to as much as fifty per cent.: and, so far as the success of the supply is concerned, far more depends on the mains than on any other part of the plant, for, unlike the generating plant, mains cannot practicably be duplicated, and, once laid, they are out of sight, inaccessible, and exposed to many deleterious, and often unknown, influences.

Mains fall broadly into three classes, viz., (1) Distributing mains, (2) Services, and (3) Feeders.

Distributing maius form a kind of buffer, or a common meeting ground, between the services on the one hand and the feeders on the other. Their office is to take the supply brought by the feeders from the generating station, to distribute it over a given area, and serve it out again at the required points to the service mains which deliver the energy to the individual consumers.

Services are the conductors through which current is supplied to any particular consumer to whose sole use they are allocated.

Feeders are mains connecting the generating station directly to a definite point on a distributing main, and to which no connection is made for any purpose at any other point.

Distributing Mains.-The conditions to be fulfilled are as follows:In the first place, it is important that the mains should be so arranged that they can be tapped at any point along their length. They should not require that connecting boxes be laid down at predetermined points, for many of these will certainly be wasted, and many will be found to be in inconvenient positions, necessitating either the entrance of the service line at an unsuitable point in the building, or the running of unnecessarily long connections. Jointing on of service lines should be easy, quickly effected, and should not require highly trained men or special contrivances. While low first cost is highly desirable, it is of greater importance that the mains should be of durable materials, and should be unlikely to fail; for a failure of