Where alternating current is available in commercial wireless telegraph
stations, the induction coil has been gradually superseded by the more
modern transformer for charging the oscillation condenser. Since the
transformer is less expensive to construct than an induction coil
capable of transmitting the same distance, it is favored by many amateur
experimenters. A one quarter kilowatt transformer has a sending range of
over 50 miles when used with a properly constructed aerial about 80 feet
high. This is probably the size best suited to the average private
installation.
The secondary of a wireless transformer seldom develops potentials
exceeding 15,000 to 20,000 volts, while those of an induction coil range
from 1 5,000 to 300,000 volts. However, the strength of the secondary
current of a transformer is so much greater than that of an induction
coil, that more powerful and penetrating waves are developed. For these
reasons a transformer is always rated by its output in watts or
kilowatts rather than by the spark length produced at the secondary
terminals. The spark of a one quarter kilowatt transformer is only 0.25
to 0.50 of an inch, while the spark of a one half kilowatt transformer
may be the same length but still represent more energy.
There are two distinct types of transformers in use, known as the "open"
and "closed," accordingly whether the core is straight like the core of
an induction coil or in the form of a hollow rectangle. The open core
type is used in the government stations and by the United Wireless
Telegraph Co. It is the simpler and more easily constructed of the two,
but is also less efficient and requires that more material be expended
to bring it up to a definite rating. In principle it is simply an
induction coil operated on alternating current minus the interrupter and
condenser. In view of the greater currents employed, the windings must
be larger and heavier than those of the induction coil to prevent
heating.
Before commencing the construction of a transformer one should read the
chapter on induction coils and use the same care emphasized there in
regard to building coils.
*Open Core Transformer.*—The transformer described below will transmit
from 20 to 75 miles and consume about 300 watts on the no volt 60 cycle
alternating current.
The core is 16 inches long and 2 inches in diameter. It is built up of
soft iron wires in the same manner as if it were the core of an
induction coil.
The primary is composed of two layers of No. 14 double cotton covered B.
S. gauge magnet wire. The primary is 14 inches long and is wrapped with
a layer of micanite cloth 3/8 inch thick to separate it from the
secondary winding.
It is never advisable to use shellacked cotton cloth as insulation. When
cotton is dried and shellacked, it is at first a good insulator, but
soon becomes capable of absorbing moisture. Shellac carbonizes at a low
temperature, and if a transformer or coil having any of this sort of
insulation entering into its construction is overheated, the insulation
is liable to become a conductor. Micanite cloth is the best insulation
for transformers. The dielectric strengths of the different forms of
micanite are shown by the following table.
The secondary is wound in ten sections over the micanite insulation.
Each section is 6 inches in diameter and 1 1/4 inches thick and is wound
with No. 30 B. S. single silk covered wire. The sections are separated
by disks of blotting paper 1/8 inch thick and 7 inches in diameter,
treated as described in Chapter IV. The completed transformer should be
placed in a box and covered with oil.
*A One Quarter Kilowatt Closed Core Transformer.* The simplest form of a
closed core transformer consists of two independent coils of insulated
wire wound upon an iron ring. When an alternating current is passed
through one of the coils, known as the primary, it generates a magnetic
flux in the iron core. The lines of force passing through the core
induce in the secondary coil an electromotive force the magnitude of
which is in nearly the same ratio to the primary inducing electromotive
force as the number of turns of wire in the secondary is to the number
of turns in the primary. For example, if it is desired to raise the
potential of the no volt alternating current to 22,000 volts, the number
of turns in the secondary of the transformer must be at least 200 times
the number in the primary.
A circular ring of iron wire presents several theoretical advantages as
a transformer core but it would be difficult to form and afterwards
place the windings in position. The core is therefore usually in the
form of a hollow rectangle, built up of very thin sheets or laminations
of soft iron carefully insulated from one another by a coat of varnish.
If the core were solid or the separate laminations not insulated from
one another, heavy currents, known as eddy currents, would be set up in
the iron and cause heating. A considerable loss in the efficiency of the
transformer would also result.
One half of both the secondary and the primary windings of a properly
designed transformer are placed on opposite sides or "legs" of the core
in order to reduce the magnetic leakage and increase the efficiency. The
only difficulty involved in such construction is the proper insulation
of the primary from the secondary, but if careful attention is given to
this point no difficulty will be experienced.
*Core.*—The dimensions and method of assembling the core laminations are
indicated by Fig. 44. Long strips are cut from soft Russian or Swedish
stovepipe iron. The strips, which are 1 3/4 inches wide are then cut up
into short lengths, one half of which are 7 3/4 inches long and the
other half 4 3/4 inches. Enough are cut to form two piles of each size 1
3/4 inches high when compressed. The completed core will then form a
hollow rectangle 9 1/2 x 6 1/2 x 1 3/4 inches.
[Illustration: Fig. 44. Assembly and Dimensions of Core.]
The strips must be dipped in some good insulating varnish such as P. &
B. compound and thoroughly dried before they are assembled. Both "legs"
(the longest sides) are laid on a table with the alternate ends
overlapping as shown by _A_ and _B_ in Fig. 44. After the short pieces
_C_ and _D_ have been slipped between the overlapping ends the whole
core is squared up. The strips _D_ are then carefully removed and one
end of the core thus left open until all the windings are in place.
Three or four layers of well varnished linen cloth are wound tightly
over the "legs" preparatory to winding on the primary.
*Primary.*—Four fiber heads, _H_, 4 3/4 inches square, 1/2 inch thick
and having a square hole 1 7/8 x 1 7/8 inches cut in the center are
required. One of the fiber heads is placed on each end of the assembled
"leg" as shown in Fig. 48.
[Illustration: Fig. 45. Fiber Head and Separator.]
The primary winding is wound in six layers, 4 1/2 inches long, three
layers on each "leg." About three pounds of No. 16 B. S. gauge double
cotton covered magnet wire are required for the winding. The terminals
of the two halves of the primary are led through the fiber heads at the
same end of the transformer. The windings are not to be carried close up
to the fiber heads but begin and end about 1/4 inch from them, so that
the remaining space may be filled by winding in a strip of micanite
cloth 1/4 inch wide. The primary and secondary windings are separated by
a strip of micanite cloth 5 inches wide, wound over both of the primary
windings close up to the heads until a layer 1/2 inch thick is formed.
*Secondary.*—The form on which the secondary sections are wound is
illustrated in Fig. 46. All the parts are cut out of wood except the
shaft and are made of the dimensions indicated. If the center of the
form is slightly tapered it will greatly facilitate the removal of the
completed sections. Sixteen sections are required. When removed from the
winder they will be in the form of hollow squares 4 1/2 x 4 1/2 x 7/16
inches.
[Illustration: Fig. 46. Section Form.]
About ten pounds of No. 34 B. S. gauge silk covered wire are required to
wind the sections. Cotton covered wire must not be used to avoid
expense, because with it a sufficient number of secondary turns cannot
be secured to bring the secondary current up to the proper voltage. By
observing explicitly the instructions and precautions given below no
trouble will be experienced in handling enameled wire and forming the
sections. The form should be placed in a lathe chuck or some other
machine which is convenient and whereby the form may be rapidly revolved
under the control of the operator.
Saw slots are cut in the wooden flanges and the center of the form as
shown in the illustration so that silk threads may be passed under and
around the completed section and tied so that a possible "cave in" of
the wire is prevented. After tying up the section should be removed from
the form by unscrewing the nut and taking off the flange.
[Illustration: Fig. 47. Methods of Connecting Sections.]
When winding the wire it must be very carefully watched for loops or
kinks and only be laid on in even layers. It must also be borne in mind
that enameled wire cannot be as tightly wound as fiber covered wire for
reasons heretofore explained. In case the wire becomes broken, it must
be smoothly spliced and soldered. Do not under any consideration use
acid as a flux or heat the wire with a flame. Acid will corrode the fine
wire, and the flame will badly oxidize or melt it. Use a short piece of
No. 8 B. S. gauge tinned copper wire set in a small file handle as a
soldering iron, and rosin as a flux. Paraffin some silk binding tape
such as dressmakers use and wrap the joint with a small piece. The
sections as they are removed from the winder must be taped and then
carefully marked with an arrow which points in the direction of the
winding.
Fig. 47 illustrates the two methods of connecting up the sections. It
will be noticed in the second method that the arrow denoting the
direction of winding points down on every alternate section. This does
not indicate necessarily that every alternate section is wound in an
opposite direction from the other, but that they have merely been turned
around so that the arrows come on a reverse side of the core and point
in an opposite direction. This precaution must be taken in order that
the current will flow through all the sections, and is made necessary
because the inside terminal of one section is connected to the inside
terminal of the adjacent section and the outside terminal of that
section is connected to the outside terminal of the next adjacent
section. The first method, _A_, illustrated in Fig. 44, is less
complicated and does not require this reversal, but for various reasons
is not to be recommended in place of _B_.
Eight of the completed and taped sections are placed on each "leg" of
the transformer, with one of the fiber separators between each pair as
in Figs. 48 and 49. When each "leg" has been completely assembled,
solder all the secondary terminals together so as to connect as in Fig.
47.
[Illustration: Fig. 48. Assembly of Leg.]
Then place the remaining fiber head, _S_, on each of the "legs" and
finish assembling the core by slipping in the end strips _D_.
Square the core up perfectly true and fasten by four fiber strips _M_,
Fig. 49, 9 3/4 inches long, 1 3/4 inches wide and 1/2-inch thick. The
strips are placed in the position shown in Fig. 46 and a hole _P_ bored
in the end of each. Four 1/4-inch bolts, two of which are 3 inches long
and two 3 1/2 inches, pass through the holes in the strips, so that when
the nuts are screwed on the core is clamped firmly. The two longer bolts
are placed at the same end of the transformer.
[Illustration: Fig. 49. Transformer with One Secondary removed.]
The terminals of the primary lead out to four binding posts mounted on
the fiber strips. The pillars which support the secondary binding posts
are fiber rods, 1 inch in diameter and 2 inches long. The lower end of
each is bored and tapped to fit the upper ends of the longer bolts which
clamp the fiber strips together. An insulating shield must be placed
between the two secondary windings to prevent sparks from jumping
across. A piece of fiber 5 x 5 x 1/8 inches will serve nicely for this
purpose. If the primary windings are placed in series the transformer
will consume about 300 watts. When the transformer is placed in a box
and the box filled with some boiled amber petroleum, the windings may be
connected in parallel and the transformer will consume about 500 watts.
It will then transmit over 100 miles providing the aerial is at least
100 feet high.
The wiring connections are diagramed in Fig. 50. A variable inductance
or reactance coil is connected in series with the primary circuit to
steady the current, as explained in the paragraph under reactance. A
reactance suitable for this transformer may be constructed by winding
two layers of No. 12 B. S. gauge cotton covered wire, six inches long,
around a hollow wooden tube made of cigar box wood. The core is built up
of soft sheet iron to form a rectangle 8 x 1 3/4 x 1 3/4 inches which
will just slide in and out of the tube. The windings should be about six
inches long.
[Illustration: Fig. 50. Wiring Diagram.]
Five half-gallon Leyden jars form about the right capacity for the
secondary of the transformer when the windings are in series. Twice that
number must be used when the windings are in parallel.
The secondary sections must always be kept in series, as otherwise the
voltage would not be high enough to properly charge the condenser.
Construction of a 2-K.W. Transformer.
*Core.*—Strips 2 1/2 inches wide are cut from soft sheet iron. One half
of them should be 11 1/2 inches long and the other half 6 1/2 inches.
Enough are cut to make two piles of each size 2 1/2 inches high. Both
sides (the longest strips) of the core are built up with the ends
overlapping as in Fig. 41.
The ends (the short pieces) are then slipped between the overlapping
ends of the long strips and the whole core squared up. The completed
core should have a cross section of 2 1/2 inches square and form a
hollow rectangle 8 x 14 inches. The strips of iron must be dipped in P.
& B. insulating varnish and dried before they are assembled.
*Primary.*—Four fiber heads 6 inches square, 1/2 inch thick and having a
hole 2 1/2 inches square are made. One of these is placed on the core
legs as shown by Fig. 48. Two or three layers of well varnished linen
are wound over the core preparatory to winding the primary. Room must be
left for the other head to be placed on the opposite end after the
windings are all in place. The primary consists of 100 double turns of
No. 12 B. S. gauge double cotton covered magnet wire. Fifty of the
double turns are wound on each leg. The four terminals of the primary
are led out through the fiber heads. Care should be taken to distinguish
them from each other so that they may be identified when the transformer
is completed. The primary is then wound with a strip of micanite or
empire cloth 8 inches wide until it measures 4 inches square over all.
*Secondary.*—The secondary coils, which are eight in number, are wound
on a form 4 inches square and 1 1/2 inches between the flanges. The
construction of the form is similar to that shown in Fig. 46 but is
larger. The slots are also necessary here so that the completed section
may be tied up.
About ten pounds of No. 30 B. S. gauge single silk covered wire are
required to wind the secondary. The sections are wound in smooth even
layers until they are six inches in outside diameter. They are then tied
up and removed from the winder. The sections are separated by sheets of
fiber 6 1/2 inches square on the outside, 1/2 inch thick, having a hole
4 inches square cut in the center. The sections are all connected in
series and the terminals soldered to strips of copper.
After the secondary coils are all in place and connected, the fiber head
is slipped on the end of the leg. Then the short lengths of the core,
which had been pulled out again after squaring the core up, are slipped
into place. The core is squared up again and fastened together by boring
a 3/8-inch hole completely through the core at each corner. Two strips
of fiber 12 x 2 1/2 x 1/2 inches are bored with corresponding holes in
their ends. These strips are placed at the end of the transformer, on
top of the core, and 1/4-inch bolts passed through the holes in the
fiber and the core. The bolts are wrapped with micanite cloth where they
pass through the core, and an insulating washer is placed under the
nuts, so that the iron core laminations are not electrically connected.
The nuts are tightened until the core is held firmly together.
The fiber strips also serve as insulated supports for the binding posts.
The copper terminals of the secondary lead to two binding posts mounted
on two fiber or hard rubber pillars 1 inch diameter and 4 inches high.
The rods are arranged as explained in the section under the heading of a
1/4-K.W. transformer.
The transformer is designed for use on a 60-cycle 110-volt current. It
may be used on 220 volts if the two primary coils are connected in
series. When the primaries are in series, and the transformer is used on
the 110-volt current, it will deliver a voltage of about 12,000 at the
secondary. With either primary alone the voltage will be about 12,000,
and with both in parallel about 25,000. It will then deliver a very
heavy current at the secondary and draw from the line about 20 amperes
in the primary. If used with a proper tuning helix, condenser and
aerial, the transformer is capable of sending about 300 miles under
favorable conditions.
If the transformer is to be used for long periods at a time, it is best
to place it in a tight wooden box 18 inches square and 12 inches deep.
The box is then filled with boiled linseed oil or amber petroleum.
A rheostat or impedance and reactance coil should be placed in series
with the transformer to regulate the current and also to prevent arcing
across the spark gap.
*Reactance.*—In Chapter I, the lag and lead of a circuit were explained
in connection with tuning. This is a property of every alternating
circuit and is brought to our notice again in the transformer which
charges the condenser. The current developed by a transformer is a
leading current, since the instantaneous values of the current do not
correspond to the proportionate values of the voltage supplying the
current. In order to force the current values of the charging current to
correspond with the voltage it is necessary to produce a "lag." This is
accomplished by means of an adjustable reactance in series with the
primary of the transformer.
A reactance or inductance suitable for the 2-K.W. transformer may be
made by building up a coil in the same manner as described under the
heading of the J-K.W. transformer. The reactance will have to be
somewhat larger on account of the heavier currents. The core is built up
of sheet iron to measure 2 1/2 x 2 1/2 x 10 inches when completed. The
coil is wound around a wooden form and is composed of about 100 turns of
No. 8 B. S. gauge double cotton covered magnet wire. By varying the
amount of core inserted in the hollow coil the energy may be adjusted as
desired.
[Illustration: Fig. 51. Clapp-Eastham 1/4-K.W. Transformer.]
Fig. 51 illustrates the 1/4-K.W. transformer manufactured by the
Clapp-Eastham Company. The core is so constructed that a small metal
tongue of soft iron projects from one side of the core towards the
opposite side between the windings, but is separated from the opposite
side by a small air gap. Several objects are accomplished by this
tongue, which gives rise to magnetic leakage; the inductance of the
primary is increased thereby to such an extent that the transformer is
self-controlling, so that it may be connected directly to the source of
alternating current supply of ordinary commercial frequencies and
potential, and the current flowing in this circuit be regulated by
varying the number of turns in the primary coil. As this magnetic
leakage gives rise to a loose coupling effect, the primary and secondary
circuits may be brought into resonance by placing a suitable capacity
across the secondary terminals. This condition of resonance brings the
power factor to a materially higher percentage. While the power factor
of the open or closed core transformer is seldom above 50%, this type of
transformer has a power factor of 80 to 90% when used with a suitable
condenser.
[Illustration: Fig. 52. United Wireless Motor-Generator set for
supplying Alternating Current to the Transformer.]
Another point of considerable advantage is the almost entire freedom
from arcing at the spark gap when this type of transformer is used. The
spark gap is connected directly across the secondary terminals of the
transformer and the condenser. The primary turns of the helix and the
spark gap are connected in series. When the transformer is in operation,
this condenser being across the secondary, the transformer is in
resonance and the condenser is charged to such a point that it will jump
the spark gap. At the instant that the spark passes, the secondary of
the transformer is practically short circuited through the spark gap. As
this circuit is now closed and the condenser out of circuit, the
secondary circuit of the transformer is no longer in resonance and the
energy immediately drops off, destroying at once the tendency for an arc
to form. As soon as the spark has passed, the condenser of course comes
in to play and the condition of resonance being reestablished the same
process is repeated. The Clapp-Eastham Company have made application for
a patent on any transformer employing this or any similar construction
for use in charging a condenser.