October 1962 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Glass-encapsulated bi-reed magnetic switches were a relatively
recent invention that came out of Bell Labs when this Carl &
Jerry episode was published in 1962. It seems like such simple
and common components have been around forever, and they have
for a growing portion of people in the electronics realm since
that was more than half a century ago. As is often the case
with John Frye's enterprising pair of teenage experimenters,
the reader is treated to a tutorial on the operational theory
of the switch, with its dependence on magnetization by induction.
The story ends up being quite humorous, and reminds me a bit
of the old
All in the Family episode where a window salesman uses
a photography light measuring meter to convince Archie he is
actually detecting severe air drafts around the windows, when
in fact it is merely registering a greater light level near
Carl & Jerry: The Difference Detector
A Carl and Jerry Adventure
By John T. Frye W9EGV
Jerry was hunched over a bench in the lab and workshop of
WCCR, the master control station of the Parvoo University campus
wired wireless system. Since he and Carl had helped Jimmy Young,
the station manager, run down a "wildcat" broadcast station
the previous semester, the two boys had been privileged to work
and experiment in the shop. Jerry grinned to himself as he heard
heavy feet pound the stairs outside; and a minute later his
chum, Carl, came puffing through the door.
"Hey! What are you trying to do ... shake me?" Carl demanded.
"No, I left a note on the tack board in our room telling
you where I was going; but, as usual, you obviously never thought
to look at it. Come on over here. I've something to show you."
"It doesn't look like much," a still unmollified Carl sniffed,
glancing down at a couple of skinny little glass capsules on
the bench. Each was about 3/4" long and 5/32" in diameter. A
stiff piece of shiny wire protruded some 3/8" from either end
of the capsule.
"I intend to make you eat those words," Jerry warned. "These
are 'bi-reed magnetic switches,' first developed by Bell Telephone
Laboratories and now manufactured by several different companies.
If you will look closely, you will see that each nickel-iron
wire is flattened out into a blade just after it passes through
the end-seal into the capsule, and that these blades extend
past each other for a short distance and are separated by a
few thousandths of an inch. What you can't see is that the opposing
surfaces of the overlapping portions of the blades are coated
with diffused precious metal - probably gold - to form contacts,
and that the inside of the glass envelope is filled with a mixture
of nitrogen and hydrogen."
"Very fascinating, I'm sure," Carl remarked as he made an
elaborate pretense of smothering a yawn.
Instead of answering, Jerry connected a flashlight bulb,
a battery, and one of the bi-reed switches in series. Next he
held a little rod of black metal about an inch long and a quarter
of an inch in diameter parallel to the glass capsule and about
an inch away from it. The bulb lighted. He pulled the metal
rod back a half inch, and the bulb went out.
"What's that little black stick?" Carl demanded, becoming
interested in spite of himself.
"Its a small alnico magnet. When I hold it close to the switch,
the two blades are magnetized by induction with opposite polarity
and attract each other into contact, lighting the bulb. When
I remove the magnet, the spring of the blades pulls them apart
and restores them to their original position."
Jerry held the magnet about a half-inch from the glass-enclosed
switch so that the bulb was lit; then he carefully slid a small
plate of soft iron between the switch and the magnet. The lamp
went dark. He removed the plate and the light came back.
"That soft iron plate 'shields' or 'short-circuits' the magnetic
field, however you want to think about it," Jerry explained,
"and allows the switch to open. I'm told that one auto manufacturer
is using, or intends to use, devices like these as limit switches
on his electrically operated windows. Current to a directional
winding of an operating motor passes through a bi-reed switch
held closed by a nearby magnet. When the window reaches the
end of its travel, a small soft-iron vane attached to the window
frame slides between the magnet and the switch, causing the
latter to open and cut off power to the 'up' or 'down' winding
of the motor, as the case may be. Neat, huh ?"
"Very neat," Carl agreed. "You could use the same system
in reverse as a burglar alarm on a house window by letting the
vane slide out of the way and close the switch, sounding an
alarm, when the window was raised. I hereby retract my slighting
remarks about your little jewels."
"Oh, there's more," Jerry insisted as he reached over and
picked up a little hollow solenoid 7/8" long and 3/8" in diameter.
The magnetic switch was disconnected, slid into the hole in
the center of the coil, and restored to its battery-and-bulb
circuit. Leads from the coil winding were connected through
a milliammeter to a variable-voltage d.c. power supply. As Jerry
slowly turned the knob to increase current through the coil,
the flashlight bulb snapped on when the current reached 4 ma.
When it was reduced to 2.5 ma., the lamp went out.
"As you've already guessed," Jerry said, "magnetism for closing
the contacts is now being furnished by the small current flowing
through the thousands of turns of that 2000-ohm coil. Our bi-reed
switch has thus been converted to a sensitive, high-resistance
relay that can, for instance, be used in the plate circuit of
a tube. Not all switches I've tried are as sensitive as this
one, but none take more than a few milliamperes of coil current
to close them."
"But that's only a single-pole relay," Carl pointed out.
"Yes, but it's a simple matter to elongate the opening in
the coil so that several bi-reed switches can be slid in side
by side to form a multi-contact relay," Jerry answered. "And
you can work another slick dodge to achieve a normally closed
set of contacts. You simply slide a little magnet in alongside
a switch to keep the contacts closed. Then the direction of
current through the coil is arranged so its magnetic field bucks
and cancels that of the permanent magnet, thus allowing the
contacts to open when the coil is energized."
"Somebody did a lot of thinking about and playing with those
switches," Carl offered.
"You're right; and no wonder: they're small and light; their
firm contact pressure makes them ideal for use in dry circuits;
their hermetically sealed contacts can be used in explosive
atmospheres without danger; and the small contact spacing makes
them very fast, with less than a millisecond actuation time."
"How about power-handling ability?" "I've no specifications
on these tiny switches, but I doubt they're designed to handle
much power. I'd say a tenth of an ampere or so would be all
you'd want to apply to the contacts. Bi-reed switches are made
in various sizes, though, and a slightly larger model is advertised
as being able to handle 15 watts into a resistive load, with
currents up to 1 ampere and voltages up to 250 volts."
At this moment Jimmy Young stuck his head into the door of
the shop. "Say, you two. I'm supposed to go over to the Women's
X Hall tomorrow night and check out the operation of the new
slave station we've just installed there. Would you foxes like
to accompany me to this chicken roost ?"
"Yes!" Carl and Jerry said in chorus. "I'll be handling the
transistorized field strength meter; one of you can make like
he's monitoring with a little receiver; but I don't know what
the other's excuse for tagging along can be," Jimmy reflected.
"He ought to be carrying something."
"Don't worry," Carl said hastily. "I'll have something to
carry. Just wait and see."
And when they met in front of the girls' residence hall the
next evening, Carl was carrying something. It was a little cube
of mahogany, about 2" on an edge, with a red-tinted flashlight
bulb sticking out the top. Burned into the other five faces
were crude reproductions of the symbols for Male, Female, Birth,
Death, and Infinity, copied straight from the beginning of the
Ben Casey TV program. Jimmy eyed this suspiciously, but before
he had a chance to quiz Carl about it they were ushered into
the house, preceded by a warning cry of "Man in the house!"
and a great scurrying up and down stairs.
Jimmy set about checking the signal strength. His indicator
was actually a small meter connected in the collector circuit
of one of the agc-controlled i.f. stages of a transistorized
receiver. As he went from room to room, the meter deflection
gave an indication of the relative strength of the wired wireless
signal. Jerry had an earphone in his ear and listened for any
interference to the program with another little receiver. Carl
brought up the rear with the mysterious little mahogany cube
balanced on the palm of his outstretched hand. The little red
bulb was dark, but Carl kept his eyes glued to it as though
expecting something dramatic to happen any moment.
Nothing did happen, though, until they moved into the lounge
where several girls were sitting around chatting and watching
TV. As Carl's blue eyes swept the scene, they settled on one
rather plain-looking girl off in a corner by herself looking
wistfully through her horn-rimmed glasses at the others. Quietly
Carl edged around the room until he was standing beside the
girl; then he gave a startled exclamation as the little red
bulb flashed brightly. "Here's one!" he called excitedly to
Conversation halted abruptly in the room as everyone stared
at Carl and the glowing red bulb. It continued to burn brightly
until he took three or four steps backward, and then suddenly
it went out. A step forward toward the startled-looking girl
in the glasses brought it back on.
"Wha-what is that?" she quavered. "It's a difference detector,"
Carl answered promptly.
"My friend and I stumbled on this physio-electronic phenomenon
a short time back," Jerry explained glibly. "While we don't
understand it completely ourselves, we know the device reacts
strongly to the difference of the sexes - if you'll pardon the
expression. When in the hands of a strong masculine type, such
as my friend Carl, the little red bulb glows brightly in the
presence of a strongly feminine type. What it reacts to is a
combination of attractiveness, charm, sex appeal, what our parents
called 'It - in short, all the qualities that make a girl a
girl. Maybe I can show you what I mean. Carl, back away until
the light just goes out."
Carl did, and Jerry reached over and gently removed the girl's
glasses, revealing a pair of lovely violet-colored eyes. Instantly
the red light flashed on.
"See how the simple act of removing the glasses enhanced
the feminine charm of this girl and increased the intensity
of her difference field?" Jerry asked.
"I-I've been thinking of getting contact lenses," the girl
By this time all the girls in the room were clustered around
Carl. One took the little block of wood from his hand, and instantly
the light went out. The block was passed from hand to hand,
turned every which way, shaken violently, and even pounded savagely
with a spiked high heel; but the light refused to come on until
the difference detector was returned to Carl's hand and he moved
to the side of the girl who was still holding her glasses in
"I hate to break up this charming little seance," Jimmy said
sarcastically, "but I promised the house mother we'd clear out
of here by ten and it's already seven minutes past; so if you
girls will kindly tear yourselves away from my assistants, we'll
The last thing the boys saw as they went out the door was
all the girls clustered around the one Carl had spotlighted
with the difference detector.
"All right, you strong masculine type, how did you do it?"
"It's really quite simple," Carl answered with a chuckle
as he juggled the little block of wood back and forth in his
hands. "The bulb is connected through a bi-reed switch to a
little flashlight battery. The switch is mounted in a shallow
groove right in the bottom of the block and is concealed by
a thin layer of veneer glued over it. Tucked through the inside
of my signet ring is this powerful little magnet, no larger
than a pencil lead and painted flesh color.
"When the bi-reed switch was aligned with the magnet and
close to it," he continued, "the lamp lighted. I could make
the lamp go on or off by moving the block very slightly in my
hand. Those symbols were burned on the outside just to hoke
it up and fit in with the story Jerry and I concocted."
"Well, you certainly gave one girl a large evening, and you
also shook up some of the hoity-toity campus queens back there
in the lounge," Jimmy reflected; "but the next time I have an
invite to a women's residence hall, remind me to leave you two
scene-stealers behind, will you?"
Posted June 8, 2015
Carl & Jerry: Their Complete Adventures is now available. "From
1954 through 1964, Popular Electronics published 119 adventures of Carl Anderson and Jerry Bishop, 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
- Electronic Eraser,
- Electronic Trap, March
- Geniuses at Work, June
- Eeeeelectricity!, November
- Anchors Aweigh, July
- Bosco Has His Day,
- The Hand of Selene,
- Feedback, May 1956
- Abetting or Not?, October
- Electronic Beach
Buggy, September 1956
- Extra Sensory
Perception, December 1956
- Trapped in a Chimney,
- Command Performance,
Education, July 1963
- Treachery of Judas, July
- The Sucker, May 1963
- Stereotaped New
Year, January 1963
- The Snow Machine, December
Education, July 1963
- Slow Motion for
Quick Action, April 1963
- Sonar Sleuthing, August
- TV Antennas, August 1955
- Succoring a Soroban,
- "All's Fair --", September
- Operation Worm Warming,
- The Blubber Banisher,
- The Sparkling Light, May
- Pure Research Rewarded,
- A Hot Idea, March 1960
- The Hot Dog Case, December
- A New Company is Launched,
- Under the Mistletoe,
- Electronic Eraser,
- "BBI", May 1959
- Ultrasonic Sound Waves,
- The River Sniffer, July
- Ham Radio, April 1955
- El Torero Electronico,
- Wired Wireless, January
- Electronic Shadow,
- Elementary Induction,
- He Went That-a-Way,
- Electronic Detective,
- Aiding an Instinct,
- Two Detectors, February
- Tussle with a Tachometer,
- Therry and the Pirates,
- The Crazy Clock Caper,