77 
Amencan Agriculturist, January 27,1923 
Improving the Crystal Set 
To ^^Tune Out'' Interference of Undesired ^Stations 
S ometimes a receiving outfit will 
be located near commercial and 
amateur stations from which consider¬ 
able interference will be experienced. 
The simple equipment described in last 
week’s American Agriculturist is suit¬ 
able for open locations, although it is 
not sufficiently selective for city use, as 
a rule. In New York City there are 
broadcast programs being sent out 
on 360 and 400 meters at the_ same 
time, and builders of single coil sets 
have found it almost impossible to 
“tune out” or eliminate the unwant¬ 
ed station. 
Should the builder of the set shown 
in Figure 1, shown in the American 
Agriculturist of January 20, be troubled 
by such interference, it is but a step 
to the arrangements of figure 3. The 
tuning coil, without any change, now 
becomes the “primary coil.” It con¬ 
sists of a cardboard tube, such as an 
oatmeal container, measuring 3% to 4 
inches in diameter, tightly wound with 
a layer of No. 20 enamelled magnet 
The remainder of the set is the same 
as the corresponding portion of figure 
1. The mounted crystal is held in a 
spring clip which is screwed down to 
the wooden base, while the “cat- 
whisker” is held hj a binding post and 
machine screw. The cat-whiskdr wire 
should be sharpened at its end, and it 
will be found that the proper pressure 
upon the crystal will vary with the 
nature of the mineral used. It is best 
to pay from 35 to 60 cents for this, 
since results depend largely upon its 
sensitivity. No. 18 bell wire is used 
for connectiops, with the insulation re¬ 
moved where contact is made. It is 
advisable to solder twisted joints to in¬ 
sure permanent contact. The phone 
condenser is supplied in various forms, 
and should have a capacity of about 
.001 micro-farads (mfds.). 
Tuning the Two Circuit Set 
The tuning of this two-circuit set is 
not complicated. First, the secondary 
coil is placed about half-way inside the 
Figure 3. A simple crystal set with secondary coil 
wire for a length of 2% inches. A 
spring brass arm is mounted on a 
brass angle to make continuous contact 
over a sandpapered arc on its surface. 
A similar cardboard tube, about _3 
inches long, although inch smaller in 
diameter than the primary, is used for 
the “secondary.” This is carefully 
wound with 45 turns of the same size 
enamelled wire, with the ends secured 
in small holes, punched at the extrem¬ 
ities of the layer of wire. Flexible 
leads, such as is used for lamp cord, is 
employed to make connections from 
ends of the secondary. If a cardboard 
box, such as a salt container, is em¬ 
ployed for the secondary, the cover 
may be left on, and the flexible leads 
brought through two holes in that. 
Figure 3 shows by dotted lines, the 
course of the flexible wires inside the 
secondary. This added coil makes it 
possible to get rid of or “tune out” 
undesired messages. 
The Variable Condenser 
In addition to tuning the primary or 
aerial circuit with the slider, there must 
be some form of tuning the secondary. 
This is done with a variable condenser, 
an instrument having two sets of inter¬ 
leaving metal plates, one set movable 
by an exterior knob. The more the 
plates are interleaved, the greater is- 
the capacity of the condenser, in just 
the same way as the capacity of the 
aerial increases When the aerial wires 
are brought near the ground. 
Variable condensers are usually re¬ 
ferred to as “23 plate” or “43 plate” 
although this is not a true indication 
of their electrical size. Capacity is 
measured by a unit called the “micro¬ 
farad.” The condenser which should be 
used for tuning this particular sec¬ 
ondary circuit should have an appoxi- 
mate capacity of .0005 micro-farads. It 
may not look exactly like the one shown 
in figure 3, but it will have two bind¬ 
ing posts which should be connected to 
the secondary and to the detecting cir¬ 
cuit as indicated. 
primary. The variable condenser is set 
at about mid-scale, and the cat-whisker 
brought down upon the crystal in vari¬ 
ous spots to locate a sensitive place 
while the slider is varied. When a 
broadcasting station is picked up, the 
detector should be adjusted for maxi¬ 
mum strength, and left. Next the sec¬ 
ondary should be moved out of the 
primary—which operation is termed 
“loosening the coupling”—and both 
slider and variable condenser adjusted. 
Extreme selectivity will occur when the 
secondary is all the way out and per¬ 
haps turned at an angle to the primary. 
The strength will decrease somewhat, 
and an intermediate ;^osition is found 
which permits little interference and 
gives plenty of audibility or sound. 
The lower portion of figure 3 is de¬ 
voted to the schematic representation 
of the pictured set. Symbols used 
should be learned in relation to their 
actual appearance, for all the parts 
shown except the detector are included 
in the make-up of tube sets. Next week 
the vacuum tube will be introduced to 
the readers. It will be in a simple but 
effective form, with a total cost of about 
$20, and with a receiving radius that 
will often approach 1,000 miles. 
When Buying Badio Supplies 
When ordering radio supplies from 
mail order catalogs, or when purchas¬ 
ing from a radio dealer, it is always 
advisable to select parts made by a 
manufacturer who has sufficient faith 
in the quality of his goods to place his 
name on them. This is particularly true 
with the crystal, the condensers and the 
head telephones. The latter should be 
stamped with a resistance of at least 
1,000 ohms per phone, and will cost 
from five to nine dollars. The sensi¬ 
tivity will vary somewhat in accordance 
with the price, and it is possible to 
secure unusually good ones for ten or 
twelve dollars. - Since these latter in¬ 
struments are used later on in more 
advanced outfits, it is certainly advis¬ 
able to buy the best.— Brainard Foote. 
Whatever else may fail 
Linking city, village and farm, crossing mountain and 
wilderness, the telephone system challenges Nature in 
her strongholds and battles her fiercest moods. 
Out on his.lonely “beat” the telephone trouble-hunter 
braves the blizzard on snow-shoes, body bent against 
the wind, but eyes intent upon the wires. 
North, south, east, west—in winter and summer, in 
forest and desert—the telephone workers guard the 
highways of communication.- Traveling afoot where 
there are no roads, crawling sometimes on hands and 
knees, riding on burros, or motorcycles, or trucks, they 
“get there” as they can. 
When Nature rages to that point where few things can 
stand against her, when property is destroyed and towns 
cut off, the telephone is needed more than ever. No 
cost is too much, no sacrifice too great, to keep the wires 
open. If telephone poles come down with the storm, no 
matter how distant they may be, no matter how difficult 
to reach, somehow a way is found, somehow in 
blizzard, hurricane, or flood — the service is restored. 
Whatever else may fail, the telephone service must 
not fail, if human effort can prevent it. This is the 
spirit of the Bell System. 
“ Bell'System” 
American'Telephone^and Telegraph CWpaH30 
And Associated Companies 
One Policy, One System, Universal Service, and all directed 
toward Better Service 
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ENUINE 
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