The Santa Radegonda station at Milan is at the present moment the
second largest Edison station in Europe. The building, which was
formerly a theatre, is well adapted for the work required; the dynamos
and engines are fixed in a deep basement, while the boilers are a few
feet above the street level, the upper floors being used as stores and
testing-rooms. The dynamos, eight in number, are of the old Edison
type, with horizontal magnets; seven of these machines are connected to
the feeders which supply the mains, and these cover the district to be
lighted on the Edison network system. The motive power is furnished by
six Armington-Sims, and two Porter-Allen engines, each connected direct
to the armature of a dynamo, the speed being maintained at the uniform
rate of 350 revolutions per minute, except in the case of the spare
engine and dynamo, which is kept turning slowly, ready to be switched
on should occasion demand. The starting or cutting-out of circuit of
these large machines requires some care. In the first place, to start,
it is necessary to insert resistance into the shunt circuit of the
dynamo, which is done by a switch; but to throw 150 horse-power into
the main circuit would be dangerous to the lamps, so that the current
is first sent into a bank of one thousand lamps used as a resistance,
and these are cut out step by step; similar care is taken when a
machine is stopped. To control the electro-motive force, which varies
greatly from time to time, hand regulation is used during the day, with
the help of the Edison tell-tale, consisting of two lamps, a red and
white one, which light up when the current is high or low; but when the
night service comes on, as it may happen that two thousand lamps may be
turned out at once, an attendant has to carefully watch the electric
regulator, and be ready to insert resistance into the field-magnet
circuits by moving a wheel connected by a shaft and bevel-gear to a
system of commutators. The principal difficulty to be overcome, in
an installation where the current is distributed over a large area,
is the regulation of the electro-motive force at the various points,
as at Milan; there are no return galvanometer wires, which are now
used in both the two and the three-wire Edison systems in the United
States. The plan devised by the company’s electrician at Milan is very
ingenious, and enables the pressure at the ends of the various feeders
to be kept practically the same, although they are of different lengths
and sectional area. In the first place, resistance was added to each
feeder to equalise the resistance in each conductor; and, in order to
provide for the varying amount of current the feeder has to supply, a
peculiar form of commutator, having a guillotine-shaped contact-piece,
was inserted in the circuit. By moving this, suitable resistance is
inserted or cut out, and the attendant, having a series of numbers, has
only to set this instrument to the number shown by the ampère meter. By
far the largest amount of current is drawn off for the lighting of the
Scala Theatre, the stage-lighting alone taking more than one thousand
lights: if these were all turned on suddenly, the other lights in the
district would be dimmed; to obviate this, auxiliary feeders have been
run, which are used only when any great increase is expected;
commutators similar to those referred to above also regulate these
feeders without any special attention. The pressure at any point in the
system is by this means easily controlled, and affords an illustration
of what is perhaps not the most economical, but is found to be the most
practicable, way of maintaining a constant potential in a district
where the amount of output of current is suddenly doubled. Fig. 18 is a
plan of the network system of conductors laid through a large portion
of the city; the conductors are in outward appearance similar to
gas-pipes, the current passing through semicircular bars of copper,
embedded both for the flow and return in the same iron tube, which is
laid underground in a shallow trench. The house-supply is drawn from
the mains, and these are connected to the feeders by means of ordinary
junction-boxes, which each contain a fusible cut-out. The bridge-boxes
allow of expansion of the line, and have connections for testing
purposes. The insulation is extremely good, mainly on account of the
favourable nature of the ground, which is chiefly gravel; no trouble
has been experienced with leakage, nor has the service ever been
interrupted. The cut-outs are of an improved Edison form, but have the
disadvantage attending all lead plugs where the current is great, in
that, to guard against accidental melting due to the heating effect of
the current, the sectional area of the lead has to be much larger than
would be otherwise necessary. In fact, these cut-outs will protect the
cable against a bad short circuit, but nothing else.
[Illustration: FIG. 18.]
In addition to the glow lamps, eighty arc lamps are worked in
derivation, two in series; most of these lamps require 45 volts,
to which 10 per cent. of idle resistance is added, constituting a
total loss of current which is extremely low for a combined arc and
incandescent system of lighting. The service commenced in 1882 with
a little over one hundred lamps, and at present there are over ten
thousand glow lamps, and two hundred arc lamps are in use. At first the
new enterprise had to struggle against very great difficulties; not
only the technical difficulties of distribution by means of a network
of feeders and mains had to be overcome, but also those arising from
the prejudices of consumers and the competition of the gas company,
who tried to deter consumers from introducing electric light into
their houses. One of these means consisted in offering to the private
consumers, resident in the district which was threatened by competition
with electricity, an agreement by which the gas company bound itself to
supply gas at 5_s._ 8½_d._ per 1000 cubic feet, instead of 7_s._ 7_d._
as charged hitherto; and even now those inside the “charmed circle” of
the electric light conductors get their gas cheaper than the public
outside. One of the reasons which accelerated the adoption of electric
light was the introduction of the Edison meter, in consequence of which
consumers could be charged exactly for the amount of light they had
received, and were relieved from paying a lump sum according to the
number of lamps fixed, which was customary in the early days of the
company. The prices at which the company now provides light, at all
hours of the day and night, are as under:—
Installation Charge per
Type of Lamp. charge per lamp. lamp·hour.
_s._ _d._
10-candle 18 0·26
16- ” 28 0·40
32- ” 56 0·80
that is, a little over ½_d._ per ampère-hour; the 10-candle lamps
requiring 0·5, the 16-candle lamps 0·75, and the 32-candle lamps 1·5
ampère.
The company lends meters for 50, 100, and 150 lamps, at an annual
rent of 4_s._ 10_d._, 7_s._ 3_d._, and 9_s._ 7_d._ respectively, and
replaces, without charge to the consumer, any lamp the filament of
which has broken, but it does not replace lamps where the glass is
broken. For arc lamps requiring 9 to 10 ampères, an annual rent of £2
must be paid for the lamp itself, and a charge of a little over ½_d._
per hour for every ampère-hour. The carbons are charged for at 1_d._
per pair, lasting for about seven hours. Now that the installation has
been in use for several years, and that the company has arrived at a
very accurate estimate of the time during which an average consumer
requires the light—about one thousand six hundred lamp-hours per
annum—it proposes to simplify the method of charging large consumers,
by omitting the initial charge of each lamp, and, instead, to charge
0·6_d._ for each 16-candle lamp-hour.
The Edison meters are based on the electrolytic action of a small
fraction of the current which passes through the meter. They are cells,
with rectangular zinc plates immersed in a solution of sulphate of zinc
of 1·054 density, the distance between the plates being a little over ¼
inch. The proportion of the current which passes through the meter to
that which passes directly into the consumer’s house is 1 to 973. The
resistance of the shunt circuit is 9·75 ohms, made up as follows: cell,
1·75 ohm; metallic portion, 8 ohms. The resistance of the metallic
portion rises with the temperature, whereas that of the cells falls
with a rising temperature; and in this manner the small variations of
resistance which might take place in the cell are counter-balanced
by the equally small variations in the resistance of the metallic
portion. A complete meter consists of two similar-sized cells of the
same resistance, placed in series. The object of employing two cells
is, that when little current is passing, as in the summer months, one
cell alone is used, and when the consumption is sufficiently large both
cells are employed, and the mean between the two indications is taken
as the basis for calculation in number of ampère-hours. The quantity of
electricity passed through the cell is calculated by the loss of weight
which has taken place in the positive plate. An employé of the society
visits every meter monthly, taking away the old cells and substituting
others freshly constructed. A book is kept in which the weights of the
new plates and those of the returned plates are entered, and on the
basis of these entries the accounts are made up. The largest plates are
those in the 100-light meter, and are intended for a maximum current
of 75 ampères in the main circuit; they are 6 inches long by 2 inches
wide. In cases where a larger amount of current is taken, the capacity
of the 100-light meter is increased by joining two or more copper
strips across the terminals of the cells. The weak point of the system
is the removal of the cells, which leaves the adjustment of the account
to be paid entirely in the hands of the Electric-Light Company; in
spite of this drawback, it is stated that there has not been a single
complaint from consumers during the four years in which the meter
system has been in use.
_Discovery of Faults._
It is evident that in so extensive a system of lighting a short circuit
now and then between the lamp wires and the earth cannot altogether be
avoided. Many of the lamps have been fitted to existing gas fittings,
and are beyond the daily supervision of the company’s officers; the
faulty place is often not easily accessible, so the first step taken is
to discover on which of the two circuits the trouble has occurred. This
is done at the station by joining two 16-candle lamps in series across
the main conductors and the point of junction between the two lamps is
connected to earth by a stout wire. As long as both circuits (positive
and negative) are perfectly insulated from earth no current flows
through this middle wire, and both lamps remain hardly incandescent;
but, if one of the circuits should be in connection with the earth, the
lamp which is joined on the other circuit will brighten up, because the
potential of the middle wire and that of the faulty circuit are both
zero, and consequently the lamp between the middle wire and the sound
circuit receives the full pressure of 110 volts. To localise the fault,
contact is made between the earth and the sound circuit by means of a
fusible plug of known melting point, say for a thirty-lamp supply. If
the fault is on a portion of the external circuit, supplying less than
thirty lamps, its fusible plug will melt as soon as the sound main is
put to earth. If, however, the fault is on a portion supplying more
than thirty lamps, the fusible plug which has been inserted at the
station between the sound main and the earth will melt instead. A
series of fusible plugs are thus tried, increasing in melting capacity
until one is found that does not go: in this case, the other plug on
the faulty portion has melted, and the consumer’s lamps on that branch
are extinguished; the position of the fault is thus localised, and
the company proceed to remedy the defect without interfering in the
slightest degree with the rest of their system.