Evening star., 1922-06-19
BY A. HYATT vERRILL.
Expert on Radio Technology.

and tho novice soon tires Of expert
menting with a crystal set and dis
cards 1t in favor Of the tube set.
20 Improve" his crystal set. he adds
q loading coil to receive longer waves.
adds variable condenser to shorten
them. installs q loose-coup1ed coil or
a vario-coupler with two or three
switches and ten to twenty contact
up set with a multiplicity of controls
and adjustments.
With every set, whether crystal or
tube. the aim should be simplicity
trolls can be reduced without losing
sharpness and differentiation in tun-
ing the better. If you wish to have
a wide range of wave length recep-
fiveness 1t Is far better to have a
very long inductance with sliders
loading coil. With slider every turn
of wire may be used. hence much
finer and closer tuning may be ac-
complished. With a two-slider coil.
variable condenser. very sharp tuning
and great variation In wave lengths
Is possible. Even slider coils have
Objectionable features. In the first
place. such a coil must be mounted
on the outside of q cabinet in order
place. the sliders. rubbing upon the
wires of the coil. grind off tiny
particles OF metal which short circuit
The turns of wire, and in the third
como worn and loss of current re-
suits. But by using a vartometer and
q variable condenser all these OBJ
5ectionA are eliminated.
Variable Condenser Useful.
Indeed few people realize what an
extremely useful and efficient device
the variable condenser Is or it's value
as a tuning instrument. With
properly arranged set, either crystal
or tube. all other tuning devices may
be eliminated. but, even without do
ing this, wonderful results may be
obtained by the proper and intelli-
gent use of variable condensers.
1t must be borne in mind that load
ing coils decrease the strength of
signals brought in, and that, when
using a coupled tuning device on the
set. the loading coil should be cou-
are to be obtained. BY adding
loading coil you are merely increas-
ing the size of the coil ordinarily
used and are adding to the adjust
ments. but by using variable con-
densers to lengthen or shorten the
wave receptiveness of the set the
number of adjustments may be de-
creased. while the normal coil size
remains constant. although, of course,
the strength OF incoming oscillations
will be somewhat decreased. If you
desire longer wave lengths the con-
denSer should be placed parallel or
across the inductance. Fig. 1, where
rs. if you desire shorter wave lengths,
1t should be placed in series with the
aerial circuit. Fig. 2. thus, by using
long inGuctance with variable
condenser in series the normally long
wave length may be cut down at will.
BY using a short inductance. the nor
mally short waves received may be
increased by condenser across the
inductance. as Illustrated in the
drawings. However, this necessitates
the use of two condensers, which
adds to the multiplicity of instru-
ments, and still further reduces the
strength Of the currents. But by so
arranging the condenser that by
merely throwing a switch 1t may be
used either In series or across. SIM
plicity is gained without reducing
tho wave length variability of the
Variometer in the circuit very fine
and accurate tuning may be secured,
Fig 4.
tai set Is it's lack of Sensitiveness and
the difficulty in securing a particu-
]arly sensitive crystal or tho same
sensitive spot on the crystal twice.
The most important thing, II crystal
sets are to be improved, is to in
crease the senitivity of the set so
that very weak signals may be re-
ceived and at the same time secure
a contact with the crystal which is
not readily Jared or vibrated off and
does not require constant puttering
and readjustment
Galena Most Sensitive.
We are so accustomed to using
galena crystals and catwhisker for
%ol I1A4 tLaA Co. h<r
realize this is by no means the most
sensitive or satisfactory type of crys
tai detector. Many other crystals
we all know possess the same prop
erty of cutting off one-half of an al
ternatng current, thus acting like a
valve. But none of these. SO far as
we know, are as sensitive as galena
under ordinary conditions.
The best known of such crystals is
carbrundum and by electrifying a
sitive as the best galena by varying
the current of electricity passing
through IT. Therefore. by adding
small battery to crystal set and
the set may be made more sensitive
and the strength of the currents in
creased as we'll (Fig. 81. In fact,
such an arrangement may be COM
pared, in a broad sense, to the radio
frequency transformer in a tube set.
Not only does the carbrundum thus
treated increase the sensitiveness of
the detector. but by its use the deli
cate cat-whisker contact may be
eliminated, for blunt steel or metal
point. such as a knitting needle or
p stiff wire, should be used for
contact and as this should be pressed
firmly against the crystal. There Is no
danger of it's being Jared loose. as
in the case of the familiar cat
whisker. By fixing this contact
point in a set screw. as shown in
Fig. 6. very fine and accurate ad-
justments may be secured. Do not
imagine that you merely have to con-
meet q dry battery to your circuit in
order to make the outfit work salts
factorily. The entire success of the
electrified crystal depends upon the
regulation of the current flowing
through it and in order to permit OF
:: fine adjustment of current poten-
tiometer must be used. This is q
simple instrument, no more complf-
Gated or difficult to make than an
ordinary grid-leak.
In principle a potentiometer Is merely
n variable resistance and any resistance
materiS, such as graphite or German
silver wire, may be used in Its con-
struction.
Simple Form Potentiometer.
A very simple form of potentiometer
which will do very well for experiments
consists Of rod Of graphite placed
between two terminals or binding posts
and provided with q simple metal clip
eoatdnE o carpenters lead pencil zone
eoekinH q carpenters lead pencil Zone
of those with a broad. nat lead) In
wster until the two wooden pieces come