The problem of electric lighting from central-stations is comparatively
easy if an area can be obtained immediately surrounding, and within a
short distance of, the station, with a right of way for laying down the
electric mains direct. This happy state of affairs has not yet been
attained, consequently the generating station has more often to be in
an out-of-the-way corner of the district to be lighted, and it would be
financially impossible to use low-tension currents with correspondingly
large mains. The difficulty has been overcome in several ways by the
use of high-tension currents in the mains, and has led to the adoption
of secondary generators or transformers of electricity, which by
induction supply a current of low potential in the house-service. The
first to make this plan a practical success was Mr. Goulard, to whom
the honour of the introduction is due, although his claims as the first
inventor have been recently upset by the decision of the Courts.
The relative economy of the supply of electricity by the use of a
transformer is clearly shown by the following diagram, Fig. 7. A, B,
and C give proportionately the area of cross section of the total mass
of copper necessary to supply 5000 16 candle-power glow lamps situated
at a mean distance of 4000 feet from the dynamo. A refers to the
Edison “three-wire” system, working at a potential or electrical
pressure of 200 volts with a fall of potential or loss of energy in the
distributing feeders of 10 per cent., average distance from dynamo 4000
feet—the usual conditions on which this system is worked. B shows the
size of conductor required for the same work in an installation based
on the transformer system, potential 1000 volts, allowing 2·5 per cent.
loss in the supply mains—only one-fourth as much as in the direct
Edison system at the same average distance from dynamo. If this loss
were increased to 10 per cent., and made equal to that in the direct
system, viz. 10 per cent., the size of the conductor would be that
shown at C.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
The graphic diagram, Fig. 8, demonstrates what the relative cost would
be with each of the three conditions just named.
Mr. Goulard’s first practical application of the secondary generator
in this country was the lighting of the Underground Railway Stations,
in 1883, from Edgware Road to High Street, the generating dynamo being
fixed at the former place. These experiments, which were made by Mr.
Kenneth Mackenzie, attracted considerable attention at the time, but
it was not until the report of the jurors to the Turin International
Exhibition in 1885 was published that companies were formed to instal
the Goulard system for lighting an extensive district.
[Illustration: FIG. 9.]
The principle underlying all transformers is that of the induction
coil invented by Ruhmkorff in 1842, but described by Faraday in his
“Experimental Researches,” published in 1831-2.
Fig. 9 is a diagram of the ordinary induction coil; on a central core
is wound a short length of thick wire called the primary, and again
over this is wound a greater length of fine insulated copper wire which
forms the secondary coil. On sending a low-pressure current from the
generator round the thick wire, a much smaller high-tension current is
induced in the secondary. A contact breaker is employed to make and
break the current, or, as in the early instruments, a commutator may be
used to produce the alternations. When used as a transformer the action
is reversed, that is, a high-tension current is passed through the
primary coil, which is composed of a wire of small sectional area, the
high-pressure main connected to the dynamo also being small as compared
with the distributing cable leading from the transformer, which, acting
as a step-down induction coil, converts the electricity into a safe
working pressure.
[Illustration: FIG. 9A.]
Fig. 9 A shows the arrangement of the two separate and complete
circuits. D is the dynamo, P the primary coil, S the secondary, and L
the lamps arranged in parallel.
It is hardly necessary to go into the technical details of the various
improvements which have led up to the modern type of transformers; they
are summarised by Mr. Kapp into two classes:—
I. Core transformers, one core and two sets of coils.
II. Shell transformers, two cores and one set of coils.
No. I. are those in which the copper coils are spread over the surface
of the iron core enveloping the latter more or less completely; and No.
II. in which the core is spread over the surface of the copper coils
forming a shell over the winding.
[Illustration: FIG. 10.]
The original Goulard and Gibbs secondary generator was of the core
transformer type, it had an open magnetic circuit and cores which
could more or less be inserted into the coils so as to regulate the
electro-motive force of the secondary circuit. The transformers were
constructed with a number of copper disks or washers; these were
placed alternately primary and secondary in a vertical frame, through
the centre of which an iron core was fixed, consisting of a bundle of
straight iron wires. The core was movable in the coil in the manner of
the well-known induction coils, and thereby the electro-motive force
of the secondary current could be adjusted. In their latest design
the coils are circular in plan and rectangular in section and are
surrounded by groups of U-shaped soft iron stampings slipped over from
both sides and held together by two circular cast-iron plates with a
central bolt. The magnetic lines of force pass through the core, in at
one end and out at the other, and are then more or less disseminated
through space; it will thus be seen that the path of the lines lies
partly in iron and partly in air, and, since air has about seven
hundred times more magnetic resistance than iron, it is evident that
the number of lines created with a given current must be considerably
smaller than would be the case if the path of the lines contained iron
only. This constitutes the improvement in the Zippernowsky-Deri-Blathy
system of transformer, which has coils similar to the Goulard, but with
the iron of the core applied in the form of a ring-shaped shell,
surrounding both coils completely. This arrangement can best be
described by comparing it to a Gramme armature, in which the copper and
the iron have changed places. Imagine what is usually the core in an
armature replaced by the primary and secondary coils, and, instead of
the winding of insulated copper wire, wind iron wire around the coils,
and one of these transformers is the result. In consequence of the
lower magnetic resistance of the Class II. transformer, as compared to
that of Class I., the electrical output obtainable with equal weights
of copper and iron appears to be considerably greater in the former
apparatus. Professor Feraris, of Turin, has published some of the
results of comparative experiments made with Classes I. and II. and
finds that the coefficient of induction is 3·6 times as great with the
latter as with the former. There are many varieties of transformers
in the market which closely resemble each other; one of the most
practical is that designed by Kapp and Snell, of which Fig. 10 is an
illustration. U-shaped stampings form the shell and the cores are laid
in the double trough. The cover of these troughs is formed from the
metal removed from the interior of the stampings, and the whole is held
together in a cast-iron frame so arranged as to allow air to circulate
through the core and round the coils. The price of these transformers
is about £4 per indicated horse-power, and the efficiency under the
best conditions, namely, with full load, is, according to Professor
Ayrton, as high as 96 per cent., and when it is doing one quarter of
the full work 89 per cent.