As we wish to write of those discoveries which led up to the invention
of the transformer, we must go back to a time, old as compared with the
modern development of electrotechnics. For the starting-point of our
observations we shall take Faraday, who, like Newton in mechanics, led
the way in the domain of electricity, and whose name stands in the most
intimate relations with all inventions for the mechanical production
of the electric current, and therefore with the later development of
electrotechnics.
[Sidenote: Faraday, 1831.]
The most important discovery for which we have to thank Faraday is
that of induction. This discovery was made by him in the year 1831,
and intimated to the philosophical world in a paper read on the 24th
November, 1831, appearing in the Transactions of the Philosophical
Society in the year 1832.
Faraday’s first induction apparatus consisted of two coils of wire, the
one being slid over the other. As he was passing the current from a
battery through one of these, he made the discovery that each time the
circuit of the coil was opened or closed an electromotive force was
created in the second coil, which caused a short gush of current or
induction current to flow, provided the circuit of this coil was
closed, as might be through a galvanometer. The peculiarity of this
induced current was, that it only flowed in the second coil during the
time the current in the first coil took to reach its normal strength
after closing the circuit, or on breaking the circuit during the time
the current took to decrease from its normal strength to zero.
This discovery undoubtedly belongs to the domain of the transformer,
induction being the physical precedent upon which the transformer is
based; indeed, a transformer is in principle an induction apparatus.
[Illustration: FIG. 1.]
Fig. 1 represents the arrangement of this fundamental experiment. The
primary coil is connected with the battery, the secondary with the
galvanometer. The primary coil, in order to obtain the best effect, is
placed inside the secondary, and on opening and closing its circuit
the needle of the galvanometer is thrown to the one or the other side
respectively.
[Illustration: FIG. 2.]
The arrangement, as in Fig. 2, made by Faraday showed itself to be an
especially effective combination for the production of these induction
phenomena. There were wound round an iron ring two separate wires of
about the same length. The one coil was brought into connection with a
battery, and to the ends of the other a pair of electrodes were
attached. The current from the battery being sent through the primary
coil, lines of force were produced which ran almost altogether in the
iron core. As the core possessed only a very small magnetic resistance,
the intensity of magnetisation was very great, and on closing the
primary circuit a strong inductive effect on the secondary coil was
produced. Faraday obtained with this apparatus the first sparks of
induction. The apparatus is all the more interesting as, although not
completely without poles, it at least forms a closed magnetic circuit.
It has much likeness to the non-polar transformer of Zipernowsky, Déri,
and Bláthy, but it may be easily shown to be not entirely poleless.
Poles mean, in electrical as well as magnetic circuits, those points
between which the greatest difference of potential exists. A current
without difference of potential can only flow in an electrical or
magnetic circuit when the loss of potential in each part of the length
of the circuit, viz., the product of resistance and current, is equal
to the gain of potential, that is the magneto-or electromotive force;
therefore a current without difference of potential requires that the
resistance and magneto-or electromotive force in each part of the
length be the same. Now the magnetic resistance of a symmetrical iron
ring is constant in all parts of the length of its magnetic circuit. In
the case in question only one half of the ring was excited, therefore
poles must have been formed at both ends of the exciting coil. The
ratio of transformation of this apparatus of Faraday’s was equal to
unity, so it had therefore no claim to the designation of “transformer.”
The induction apparatus of Faraday in its simplicity was in a certain
measure the embryo out of which all dynamos and transformers have
developed. We have seen how the first induction current was discovered
by making and breaking the current from a battery in the primary coil.
This method was at first adhered to, until Faraday remarked that when
the secondary was quickly drawn out of or put into the primary coil,
induced currents were also produced without requiring to break the
circuit, the wires of the secondary coil cutting the lines of force in
the magnetic field of the primary coil. He then replaced the primary
coil and battery by a permanent magnet, which was likewise dipped into
the induction coil, Fig. 3.
[Illustration: FIG. 3.]
[Sidenote: Henry and Page, 1836.]
From this, and from the later development of this invention, it follows
that the question was not of a transformer in the present sense of
the word, but of a secondary generator. Transformers as at present
understood were first known in Europe as the Ruhmkorff’s induction
coil. Before we take up this invention we shall mention a much earlier
and like invention, which had already been made in the United States in
the year 1838. This was the induction coil of Professor Page, and was
the outcome of another invention by Professor Henry, whose apparatus
was only a single induction coil. The first public notice of Professor
Page’s apparatus appeared in the Silliman-Journal of 12th May, 1836,
under the title, “Methods and trials of obtaining physiological
phenomena and sparks from a heat engine by means of Professor Henry’s
apparatus.” In May, 1837, Sturgeon published, in the “Annals of
Electricity,” in London, a description of the apparatus of Henry and
Page.
[Sidenote: Callan, 1837.]
Callan, an English student of physics in Minnoth, showed first, in the
year 1837, that if high tension was wanted, it was necessary to employ
thick wire for the primary and thin for the secondary coil. Before this
time wires indeed of different lengths, but of equal cross sections,
had always been employed. His apparatus was not so bad as those before
known, but still stood far behind that of Professor Page.
[Sidenote: Page, 1838.]
The arrangement of Professor Page’s apparatus, which is shown in Fig.
4, was as follows:—Two coils of wire well insulated from one another
were wound on to a bundle of iron wires. A self-acting contact-breaker
was put into the primary circuit, and consisted of a double lever E,
having on one of its arms two parts bent downwards, so as to dip into
two mercury cups. The movement of the part H, as compared with that of
E, was so small that it remained always in the mercury. At M, however,
when the lever was set in motion contact was broken and made. To prevent
oxidation Page poured in a layer of alcohol over the quicksilver.
[Illustration: FIG. 4.]
The continuation of the lever in the other direction of the axis, which
was borne by two pillars K, was bent backwards, and on its end carried
a cylindrical piece of iron standing before the end of the bundle of
iron wires. If the primary coil were now placed in connection with
a source of current, the iron core became magnetised, attracted the
cylindrical piece of iron to itself, and by raising the lever E broke
the contact at M. The iron core then lost its magnetism, released the
iron armature, and the play began anew. A counter-weight F, which
could be shifted along another lever O, allowed the play of the
contact-breaker to be regulated. It will be found that this interruptor
was very like that constructed many years afterwards by Léon Foucault.
The effects which Page produced by means of this instrument were
much more intense than those produced by Ruhmkorff with his, as Page
succeeded with only a single Grove element in inducing in the secondary
circuit such a high electromotive force as produced sparks 4½ inches
in length through a vacuum tube—a result that Ruhmkorff, although his
invention created such a great and well-deserved attention, did not
attain. In the year 1850 Page built a much larger apparatus.
In order to give some idea of the magnitude of the electro-magnetic
forces which came into play here, suffice it to say, that the exciting
coils could hold suspended in the air in their interior an iron core
weighing 520 kg. The primary or magnetising coil was of square copper
wire, with a side measuring ¼ inch, and a battery of 50 to 100 Grove
elements was employed, the immersed area of the surface of the plates
being 100 square inches. This apparatus gave sparks of great length.
When, with maximum currrent strength, the primary circuit was broken,
sparks of 8 inch length were received.
[Sidenote: Ruhmkorff, 1848.]
Ruhmkorff constructed, in the year 1848, the so-called spark-inducer
named after him, the object of which was also to convert currents of
low tension into hose of very high tension. With this coil and like
coils of larger dimensions effects were produced, but only such as
were afforded by the common forms of frictional electrical machines.
All things considered, it is not a little surprising that while the
invention of the Rhumkorff coil was still in its infancy, the wonderful
output of Page’s apparatus was still, even in the year 1851, quite
unknown in Europe.
[Illustration: FIG. 5.]
Fig. 5 represents the earlier form of the Ruhmkorff apparatus. It
consisted of a bobbin of good insulating material; thoroughly dried
wood, or better, hard rubber. The two end pieces of the bobbin were
usually made of grooved glass discs, and were bound down to the
bedplate of the apparatus by two wires. Inside the coil was the already
often-mentioned bundle of iron wires. The primary or inducing wire
was next wound upon the bobbin. As this wire had to carry currents of
comparatively great strength, it consisted of only one or a few layers
of thick wire. The circuit of this coil was completed as far as two
terminals on the bedplate, first passing through an interruptor like
what has already been described. Over the primary coil, and after a
sufficient layer of insulation had been added, the secondary wire was
wound. As this wire was destined for very small currents, it was of as
fine wire as it was possible to wind. In order to obtain high potential
it was necessary that the secondary should possess many turns. In the
earlier coils a length of between 8 and 10 kilometres was used; in the
coils now made this length has been increased to between 50 and 70
kilometres. The ends of the secondary coil were connected to terminals
insulated on glass pillars. It was not nearly sufficient insulation
for the secondary wire to be covered with silk, but every layer was
well soaked with dissolved shellac, and then well dried as it should
be. A condenser in connection with the primary coil was placed under
or in the bedplate, which was usually a box. This condenser was, and
is still, often made thus:—On both sides of a strip of paraffined
paper, several metres long and of convenient breadth, tinfoil is stuck,
at the same time leaving a sufficient margin of paper for insulation.
The whole is then folded together suitably. The effect of the coil is
substantially enhanced when the sheets of tinfoil are each connected to
the circuit of the primary coil in such a way that the condenser is in
shunt to the interruptor.
In Fig. 6 is shown a newer form of Ruhmkorff’s coil, with an
interruptor like the mercury contact-breaker which we have before
described. According as the movable weight is raised or lowered, the
oscillations of the lever, and consequently the induced currents,
follow one another more slowly or more rapidly.
[Sidenote: C. T. and E. B. Bright, 1855.]
We find a further development or modification of the invention of Page
and Ruhmkorff, patented by the brothers C. T. and E. B. Bright on 21st
October, 1852, and No. 2103 in the year 1855. In the latter of these
patents the inventors state what follows concerning the nature of their
inventions.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
“A section of an induction coil made after this manner is shown in fig.
7, having a very strong effect. The primary wire, of which only a part
is shown, is wound on an iron core, and outside is surrounded by an
iron cylinder. Both of these are metallically connected by the flanges
of the bobbin, which also are of iron. The secondary coil may also be
surrounded by an iron tube, and if the resistance of the circuit be
extraordinarily great with still more primary coils, or it may be also
contained in the same tube as the primary. In cases where it is found
necessary to increase the quantity of the electro-magnetic effects, we
find that the forms shown in Figs. 8 and 9 are very effectual, and may
by varied on the same principle. The iron core in the middle is wound
with the primary wire, and is surrounded by the other iron cores, which
are fixed into the large flanges of the middle core, and carry the
secondary coils. Should still greater effects be required, more primary
or secondary coils connected in series with the others may be added
outside, in order to produce a greater extension of the poles and a
more extensive induction.”
[Illustration: FIG. 9.]
This patent is interesting also for the fact that in it we find a
disposition of parts, viz. the arrangement of several induction coils
in ranks, and connected with one another in parallel, which nearly 30
years later was taken up and practically used by Gaulard.
[Sidenote: Harrison, 1857.]
Among the patents of the year 1857 there is an English one by Harrison,
claiming as its object the passing of a primary current through one
or more induction coils, and the connection of the secondary coils
with the carbons of an arc lamp. There is nothing remarkable in the
description.
[Sidenote: Jablochkoff, 1878.]
The last attempt to use induction coils for industrial purposes is
met with in the year 1878. In this year Jablochkoff took out a German
patent, which was also carried out in practice. He required currents of
very high tension to feed his kaolin lamp; at that time such currents
could only be produced by induction coils. He writes as follows in his
patent:—
“Die Herstellung einer elektrischen Beleuchtung nach
meinen System begreift eine Serie von Induktionsrollen
in sich, wovon die inneren Drähte in eine elektrische
Leitung eingeschaltet sind.”
[Illustration: FIG. 10.]
Jablochkoff used intermittent direct currents as well as alternating
currents. The arrangement shown in Fig. 10 was for the former. He
states concerning this:—
“In diesem Falle sind die Induktionsrollen mit
Unterbrecher und Kondensator ausgestattet, oder man
kann auch, wie die Zeichnung nachweist, einen und
denselben Unterbrecher für alle Rollen anwenden. Die
Induktionsrollen B^1 B^2 B^3, nach einen beliebigen
Prinzipe konstruirt, sind in der Nähe der Lichtherde
angebracht.”