A tuning coil is merely a variable inductance wound in single layer on a
suitable form.
Fig. 121 illustrates a double slide tuner. The base is a piece of hard
wood, 12 inches long, 1 inch thick and 5 1/2 inches wide. Two wooden
heads 4 x 4 x 3/4 inches support the form upon which the coil is wound.
[Illustration: Fig. 121. Double-slide Tuning Coil.]
The form is a piece of wooden curtain pole, 9 inches long and 3 inches
in diameter. Some may prefer to use a cardboard tube in place of the
curtain pole. A tube can be made by winding a long strip of cardboard 9
inches wide around a suitable form and cementing the layers together
with shellac. The liberal use of shellac will stiffen the tube and cause
it to better retain its shape. The tube is held tightly between the two
heads by means of a brass rod which passes through the center and is
clamped by two nuts.
A square brass rod 10 1/2 inches long is fastened to the center of the
top of the heads and a similar rod to the center of the front face.
[Illustration: Fig. 122. Sliders.]
Fig. 122 shows two forms of sliders. The first one is the better and to
be preferred. A short square brass tube, _S_, fits snugly upon the
square brass rod, _R_. It cannot turn around but is free to slide back
and forth. A strip of spring brass, _C_, is soldered to the lower face
of the square tube. It is bent in a double turn and a punch mark made
near the lower end as shown in the illustration. The indentation is made
with a center punch, but should not be deep enough to break through the
metal.
[Illustration: Fig. 123. Double-slide Tuning Coil Circuits.]
The little projection on the under side of _C_ caused by the punch mark
is the only part of the slider which should make contact with the wire
on the tuning coil. It should slide easily but firmly along the wires
and touch only one at a time. Long distance signals will be considerably
weakened if the slider touches more than one wire at a time and
short-circuits a turn.
[Illustration: *Plate IV. Receiving Circuits. (Straightaway Aerial.)*]
The slider, _B_, is similar to _A_ except that it has a short length of
brass tubing, _T_, soldered to the under side of _S_ in place of the
brass strip, _C_. A small ball bearing which just fits the bore of the
tube is pushed down into contact with the wire by means of a small
spiral spring inside of the tube. Both sliders are fitted with a hard
rubber handle so that they may be adjusted without the ringers coming
into contact with the metal.
[Illustration: Fig. 124. Murdock Double-slide Tuning Coil.]
[Illustration: Fig. 125. United Wireless Receiving Set.]
Two good circuits employing the double slide tuner are given in Fig.
123. Plates IV and V illustrate the oscillation or tuning circuits of
the most prominent receptor systems.
If a loop aerial is used, more than one tuning coil is necessary as
shown by the loop aerial oscillation circuits in Plate V.
Figs. 125 and 126 illustrate the instruments employed for receiving by
the United Wireless Company.
[Illustration: Fig. 126. United Wireless Portable Receiving Set.]
In Fig. 125 the handles which are attached to the sliders of the tuning
coils project through long slots cut in the top and one side of the
cabinet.
The tuning coils in the portable outfit are mounted in a vertical
position in back of the aerial switch.
*Loosely Coupled Tuning Coil.*—By the use of a loosely coupled receiving
tuner or transformer, the range of a station is considerably increased,
as is also the strength of the signals, and much finer tuning and
selectivity made possible.
Fig. 127 illustrates the construction of such an oscillation
transformer.
The base is wood and measures 14 x 5 1/2 x 1 inches. The primary winding
is wound on a cardboard or fiber tube 4 1/4 inches long, having an
internal diameter of 2 3/4 inches and an external diameter of 3 inches.
The heads, _L_ and _M_, are the same size as those of the double slide
tuning coil. The head, _L_, has a circular hole 2 3/4 inches in diameter
cut in the center in order to permit the secondary coil to slide in and
out of the primary.
[Illustration: Fig. 127. Oscillation Transformer.]
The secondary coil is a piece of round curtain pole 2 1/2 inches in
diameter and 3 inches long. A 5/16-inch hole is bored through its axis.
The head, _K_, of the secondary coil is 3 3/4 x 3 3/4 x 3/4 inches. A
ten-point switch on _K_ is so connected that it divides the secondary
into ten equal parts and permits any number of the divisions to be used
as desired.
[Illustration: Fig. 128. United Wireless Receiving Transformer.]
A wooden post, _J_, 2 1/2 inches high and 1 1/4 inches wide, supports
one end of a 1/4-inch brass rod upon which the secondary slides back and
forth.
[Illustration: Fig. 129. Details of Receiving Transformer.]
No. 24 B. S. gauge copper wire may be used for winding both the primary
and secondary. It is also the proper size to use on the double slide
tuner. The best method is to use bare wire, wound with a thread so that
a thread is interposed between adjacent turns of the winding. Give the
whole winding one or two coats of thick shellac and allow it to harden.
Then use a strip of sandpaper to remove the shellac in a long narrow
path immediately below the sliders so that they may make contact with
the wire.
[Illustration: Fig. 130. Slider for Loose Coupler.]
Some may prefer to make a loosely coupled tuner in which the inductance
of both coils is adjustable by means of a sliding contact. In such a
case the slider on the secondary coil must be constructed as illustrated
in Fig. 130. The contact is long and narrow so that it can touch the
innermost turns, when placed within the primary. By slipping the slider
off the end of the rod and reversing it, the contact can be made to
touch the turns next to the head. The square brass rod is set in a notch
cut in the coil head so that the rod is flush with the top.
When tuning a receiving transformer, place both variable condensers in a
halfway position and adjust the sliding contacts, first on the primary
and then on the secondary, until the signals are the loudest. Then
adjust the condensers.
[Illustration: Fig. 131. Loosely Coupled Tuning Circuits.]
To cut out an undesirable station, vary the coupling between the two
coils by sliding the secondary away from the primary. When several turns
on the secondary seem to give the same results also vary the coupling.
[Illustration: Fig. 132. Combination Loosely and Closely Coupled Tuner.]
Fig. 132 illustrates the wiring diagram of a combination loosely and
closely coupled tuner. Two sliding contacts are placed on the primary
coil of the receiving transformer and connected with a double pole
double throw switch as in the diagram. When the switch is thrown on
contacts 1 and 2, the primary is connected to the detector as a double
slide tuner, and when on 3 and 4 both the primary and secondary are
brought into use as a transformer.
This arrangement may seem cumbersome and is recommended only as a
convenience in experimenting. A loosely coupled tuning coil is capable
of exact tuning, and unless one understands how to use it, he may not
hear a station because the tuner is not properly adjusted. By using the
double slide tuner first and then throwing the switch so as to tune in
on the transformer, this difficulty may be eliminated.
[Illustration: Fig. 133. Clapp-Eastham Loose Coupler.]
*Potentiometer.*—A potentiometer is not properly classed under the
heading of tuning coils, but the construction may be made so similar
that it well appears here.
The potentiometer is merely a variable resistance shunted across the
terminals of the detector battery in the manner illustrated in the
numerous detector circuits. It is used to reduce the voltage of the
battery to a value slightly below the critical voltage of the detector.
The critical voltage of a detector is the voltage at which its action
commences. In the case of an electrolytic detector, it is the voltage
required to break down the thin film of gas which collects on the "bare
point."
[Illustration: Fig. 134. A Highly Efficient Form of Loose Coupler.]
In construction, the potentiometer illustrated in Fig. 135 is in reality
a small edition of a double slide tuning coil. It is wound with No. 28
B. S. gauge German silver wire. Three binding posts are mounted on the
base, two of them connecting with the ends of the coil and one with the
sliding contact.
[Illustration: Fig. 135. Potentiometer.]
In a finely balanced circuit where long distance work and close tuning
are desired, the potentiometer must be non-inductive.
[Illustration: Fig. 136. Amco Potentiometer.]
This may be accomplished by using two potentiometers wound in opposite
directions from one another and connected in series. The two terminals
of the windings are then connected across the battery and the sliding
contacts led to the detector.
[Illustration: *Plate V. Receiving Circuits.*]