August 1962 Popular Electronics
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Carl & Jerry: Electronic Eraser
By John T. Frye W9EGV
Carl, Jerry, Police Chief Morton, and a pale, young, well-dressed
stranger were engaged in serious conversation in the boys' basement
"I need rather specialized help in a hurry," young Mr. Adams
was saying. "When I talked to the department head .. of the
Federal agency for which I work, explained the situation to
him, and told him where I was, he remembered that you boys had
helped another of our operators about a year ago. He says you
designed a small transmitter that fitted inside a dog's collar
and enabled our agent to get some valuable information."
"Sure; that was Mr. Cody, a big fellow who was light on his
feet and gave out next to nothing in the way of information,"
"Sounds like Cody," the pleasant stranger said with a wan
smile. "Anyway, I was told to contact you through Chief Morton
and see what you could do. I'm afraid, like Mr. Cody, I can
only divulge just the information you must have to understand
the problem; but believe me, this affair is important to every
"Tonight at ten a man will board a bus leaving here for Center
City. He will be carrying a small tape recording, probably in
a briefcase. On the bus will be another man waiting to receive
the tape. After it changes hands, the receiver of the tape will
somehow check the recording to make sure it's what he wants.
If he is satisfied, a large sum of money will be given to the
man who surrenders the tape.
"Now it's most essential, first, that the men do not know
they are under surveillance; second, that the tape not change
hands; and third, if at all possible, that the tape itself be
destroyed. Can you men, with your background of electronics,
think of any way these three objectives might be accomplished
"Am I right in believing it's the material recorded on the
tape that you don't want to change hands rather than the physical
tape itself?" Jerry asked.
"That's right, but what are you driving at?"
"I was thinking of some method by which we might erase the
tape without its ever leaving the possession of the man who
has it now."
"And without his knowing that it was being erased?"
"Yes," said Jerry. "I'm not at all sure it can be done, but
suppose you give Carl and me about three hours - until 8 p.m.,
say - to do some experimenting. In the meantime, perhaps you'd
better be thinking up a second plan to use in case we strike
"I like the way you think - " Mr. Adams started to say as
he rose to his feet, but suddenly he gasped and nearly doubled
over in pain. "Guess I got kind of a catch in my side," he explained
as he straightened up. "I'll be back at eight to see what you've
found out, but I surely hope you come up with something. I don't
seem to be thinking too well today, and I ran out of ideas before
I came here."
Sounds to me as though you're biting off quite a chunk, old
buddy," Carl observed after the other two had left.
"That business about the men not knowing any hanky-panky
is going on while their precious tape is being erased is the
toughie. What do you have in mind? Rubbing a big permanent magnet
over the guy's briefcase?"
"It's worth trying," Jerry answered promptly. "Let's do some
experimenting and see what kind of results we can get with brute
force d.c. erase - that's the name applied to using a permanent
magnet to erase a recording from a tape."
It didn't take the boys long to convince themselves that
this approach showed little promise. A very strong alnico magnet
out of a speaker had to be almost in contact with the tape to
erase the recording completely. When a quarter-inch of space
was kept between the magnet and a recorded reel of tape, only
a very slight weakening of the recording was noticed.
"I was afraid of that," Jerry observed.
"We must be able to work through a distance of at least a
quarter of an inch to allow for the thickness of the leather
in the man's briefcase, a cardboard container for the tape,
etc. You see, a d.c. erase system on a tape recorder doesn't
consist simply of a method of passing a tape over a strong permanent
magnet. That's only the first step, which leaves the tape in
a very strongly magnetized condition. It must then be passed
over a weaker magnet of opposite polarity, or sometimes several
magnets of alternating polarity or a magnet with a diagonal
gap in it, so that the end result is a tape which is completely
demagnetized. That's what we need, some way of literally bathing
the recording right through the briefcase with an alternating,
diminishing magnetic field."
"And how do we do that?"
"Let's try to construct a portable, battery-operated bulk
eraser. As you know, a bulk eraser is simply a strong electromagnet
with 60-cycle a.c. going through its coil. As a reel of tape
is subjected to this strong reversing field and then gradually
withdrawn, the little magnetic particles of the tape are first
jerked violently in unison out of their recorded pattern and
then are subjected to weaker and weaker pushing and tugging
forces that eventually leave them lying in the unpatterned disarray
of virgin tape."
"Don't you think it's going to be a little difficult to get
a.c. out of your batteries - or are you going to use a.c. batteries
"Don't be cute. We can get a first cousin of a.c., pulsing
d.c., simply by using an interrupter between the batteries and
our coil," Jerry retorted. "Suppose you see what you can do
about converting this old 6-volt auto radio vibrator into a
simple interrupter while I get started winding a magnet."
Jerry started by boring a small hole in the center of a 2"-diameter,
1"-thick piece of soft iron; a bolt passed through this hole
was able to hold two circles of plywood clamped against the
sides of the iron core. Next, he cut several short pieces of
plastic electrician's tape and looped them down into the space
between the two pieces of plywood with their sticky sides turned
away from the wood and iron. Then he chucked the end of the
bolt in his electric drill and wound layer after layer of #20
enameled wire on the core.
Jerry did his best to wind the wire in tight, even layers as
the electric drill rotated the core, and he kept at it until
his bobbin of wire was some five or six inches in diameter.
Next, he carefully lapped the loose ends of each piece of
tape over each other to hold the wire securely in place while
he removed the wooden sides and pushed the iron core out of
the middle. Now, very carefully, he wrapped the whole doughnut
of wire with thin plastic tape and then forced the soft iron
core back into the center.
By this time Carl had the vibrator reworked, and it was connected
to a battery eliminator and the coil so that the heavy contacts
of the vibrating reed connected and disconnected the six-volt
d.c. supply to the coil at a rate of about 115 cycles per second.
A capacitor substitute box was connected across the points of
the interrupter and adjusted for minimum arcing; then a capacitor
of this value was soldered in place.
Finally, the boys placed an issue of Popular Electronics
on top of the coil, laid a reel of recorded tape on top of that,
and turned on the battery substitute. The tape was slid around
on the magazine for a few seconds and then lifted away before
the power was cut off. When the tape was placed on the recorder
and run through in the "Play" position, not a sound - except
for a slight hiss - came from the recorder speaker.
"That does it!" Jerry exclaimed jubilantly. "Now let's get
busy installing this rat's nest and some parallel heavy-duty
six-volt lantern batteries in that old attache case of Dad's
upstairs. We want plenty of current, for the magnetic field
is dependent on ampere-turns. Mr. Adams should be here in an
hour, and I want to be able to give him a convincing demonstration."
First they cut a hole in the side of the case so that one
side of the coil could be mounted flush with the surface of
the leather. This opening was concealed with brown press-on
paper that was used to cover the entire case. The batteries
were anchored in place, and the vibrator was wrapped in several
layers of sound-insulating foam rubber. Wires were run through
the handle to a simple push-button switch which could be operated
easily with one finger.
"We want to have our magnet just as close as possible to
that reel of tape," Jerry said as he smoothed the thin paper
over the magnet core. "The strength of a magnetic field is inversely
proportional to the distance, and it works on a square law.
Plus or minus a sixty-fourth of an inch might spell the difference
between success or failure in our completely erasing that tape."
He was interrupted by the arrival of Mr. Adams and Chief
Morton. The former looked even paler than he had that afternoon,
and bent over a little.
Quickly the boys explained what they had done. They let Mr.
Adams hear a few feet of recorded tape and then placed the reel
of tape in a briefcase. While Jerry walked around the basement
dangling this case from one arm, Carl brushed against him with
the attaché case. In spite of what seemed to be a very brief
and casual contact between the briefcase and the attaché case,
the tape was found to be completely blank when it was placed
on the recorder.
"Guess we better show you how to work it; we haven't much
time," Jerry offered.
"Boys, I have bad news for you," Mr. Adams said as he wiped
cold sweat from his forehead. "You're going to have to use that
gadget yourself. The doctor is waiting for me in a squad car
outside, and I must go straight from here to the hospital for
an emergency appendectomy.
"This is a picture of the man who will have the tape," he
went on. "You should be able to spot him in the bus station.
And here are two round-trip tickets for the bus to Center City.
With that, Chief Morton half-led, half carried Mr. Adams
through the door and up the outside basement steps.
Meanwhile, Carl and Jerry stared at each other in pop-eyed
amazement. "Well," Carl finally said, "I guess we better get
started. You wrap up in your cloak while I dash over and get
I Spite of their joking, both boys had a feeling of rising
excitement as they parked their car across the street from the
They spotted their man as soon as they entered the door,
even though he was not wearing dark glasses, had no beard, and
looked amazingly like any ordinary businessman. But he also
happened to look exactly like the picture in Jerry's shirt pocket,
and a thin briefcase rested on his knees. Out of the corners
of their eyes, the boys could actually make out the square outline
of the cardboard container for a 3" reel of tape through the
It seemed to Carl and Jerry the bus would never come, but
finally it did. As the stranger stood up with his briefcase
in his right hand, Jerry transferred the attaché case to his
left hand and followed the man closely. At the door of the bus
there was a little knot of people awaiting the alighting passengers
before they started getting on, and it was here Jerry had his
While Carl stood on the man's left and asked him something
about whether the bus was on time or not, Jerry pushed the switch
under his finger and brushed his case lightly against the briefcase,
rubbing it around with a circular motion. The man never noticed.
The bus was not crowded. Carl and Jerry followed their man
to the rear, where he selected a seat alongside of a short fat
man who was dictating letters in a low voice into the microphone
of a portable tape recorder that was resting on his knees.
The two men apparently took no notice of each other until
the bus was a dozen miles out of town. Then the man with the
briefcase turned to the other and said, "Pardon me, sir, but
I wonder if you might do me a great favor. I have a tape here
sent me by my nephew in the army overseas, but I have no recorder.
Could you possibly let me hear it on your machine ?"
"Why, certainly; just let me have it and I'll get it started
for you," the fat man answered obligingly.
As Carl and Jerry watched tensely, the fat man took the tape
from the briefcase and put it on the machine. Then he put the
listening earpiece in his ear and started the tape moving. After
a few seconds he began to frown and he threw a switch that sent
the tape whirring at a faster speed. Next he turned the tape
over and tried to listen to the other side.
Finally he handed the earpiece to the first man and said
coldly, "Your nephew is a practical joker. There is absolutely
nothing on that tape. Hear for yourself."
The man trying to sell the tape listened to every inch of
it with growing puzzlement and dejection. "I don't understand
it; I don't understand it at all," he muttered over and over.
"You'd better tell your nephew practical jokes can be dangerous,"
the little fat man said menacingly as he turned his back and
stared out the window.
At Center City Carl and Jerry followed
their man outside the bus station. As he walked past an alley,
he impulsively tore the reel of tape from his pocket and tossed
it into an ash can. The boys retrieved it and took the next
Chief Morton was waiting for them at the station, and the
three of them went to the hospital. There Mr. Adams was out
from under the anesthetic, and he was delighted at what the
boys told him.
"Well done!" he said weakly. "If this keeps up, we're going
to have to put you on our payroll. Seriously, though, you'll
hear more about this as soon as I'm back on my feet and send
in my report. In the meantime, boys, you've done a great service
for your country - greater than you realize."
"As far as I can see, all we've done is get tangled up in
some government tape," Carl said jokingly as they headed for
Carl & Jerry: Their Complete Adventures is now available. "From 1954 through 1964,
Popular Electronics published 119 adventures of Carl and Jerry, two teen boys with a passion for electronics and a knack
for getting into and out of trouble with haywire lashups built in Jerry's basement. Better still, the boys explained how
it all worked, and in doing so, launched countless young people into careers in science and technology. Now, for the first
time ever, the full run of Carl and Jerry yarns by John T. Frye are available again, in five authorized anthologies that
include the full text and all illustrations."
Carl & Jerry Episodes on RF Cafe
- Carl & Jerry: Anchors Aweigh, July 1956
- Bosco Has His Day, August 1956
- The Hand of Selene, November 1960
- Feedback, May 1956
- Abetting or Not?, October 1956
- Electronic Beach Buggy, September
- Extra Sensory Perception, December
- Trapped in a Chimney, January 1956
- Command Performance, November 1958
- Extracurricular Education, July
- Treachery of Judas, July 1961
- The Sucker, May 1963
- Stereotaped New Year, January 1963
- The Snow Machine, December 1960
- Extracurricular Education, July
- Slow Motion for Quick Action,
- Sonar Sleuthing, August 1963
- TV Antennas, August 1955
- Succoring a Soroban, March 1963
- "All's Fair --", September 1963
- Operation Worm Warming, May 1961
- The Crazy Clock Caper, October 1960
- Two Detectors, February 1955
- Tussle with a Tachometer, July 1960
- Therry and the Pirates, April 1961
- The Sparkling Light, May 1962
- Pure Research Rewarded, June 1962
- A Hot Idea, March 1960
- The Hot Dog Case, December 1954
- A New Company is Launched, October 1956
- Under the Mistletoe, December 1958
- Electronic Eraser, August 1962
- Blubber Banisher, July 1959
- "BBI", May 1959
- Ultrasonic Sound Waves, July 1955
- The River Sniffer, July 1962
- Ham Radio, April 1955
- El Torero Electronico, April 1960
- Wired Wireless, January 1962
- Electronic Shadow, September 1957
- Elementary Induction, June 1963
- He Went That-a-Way, March1959
- Electronic Detective, February 1958
- Aiding an Instinct, December 1962