Carl & Jerry: The Difference Detector
October 1962 Popular Electronics

October 1962 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, 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 a window.

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 Jerry.

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 said shyly.

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 her hand.

"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 be going."

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?" Jimmy demanded.

"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