Monthly Archives: March 2017

1957 Combination Radio-Geiger Counter


Sixty years ago, prospecting for uranium was a popular pastime, and the gentleman shown here in the March 1957 issue of Popular Mechanics was doing it right. The accompanying article noted that prospecting could be a lonely proposition, and this project allowed you to bring along a companion who would assist and entertain you without demanding a portion of your claim: You could build this combination radio-Geiger counter!

The basic circuit was a superheterodyne radio, using four tubes: 1R5, 1U4, 1U5, and 3V4. The Geiger counter used a 1B85 Geiger tube from the Victoreen Instrument Company. The filaments ran off two cells, and a 45 volt battery supplied the B+. Since 45 volts wasn’t quite enough for the Geiger tube, the circuit used an transformer and spark-gap circuit to charge up a capacitor to a higher voltage. The is was accomplished with a pushbutton in series with the transformer’s primary. You would pump the pushbutton for about 30 seconds to charge the capacitor, which would allow a sufficiently high voltage for the Geiger tube.

The output of the geiger counter went to the audio section of the radio, and when you hit uranium, you would hear the clicks from the speaker.

In the diagram below, the radio is shown in black and Geiger counter in red.  The magazine also noted that if you already had a suitable portable radio, you could add the Geiger counter circuit.



Carbon Button Microphone Amplifier


While they were rarely used in radio applications, the diagram here shows how a carbon button microphone amplifier could be used to drive a loudspeaker from a crystal set. This diagram is from 90 years ago, and appeared in the 1927 British Radio Year-Book.  The diagram actually appears in the advertisement for a book entitled Successful Crystal and One Valve Circuits by J.H. Watkins, who according to the ad was the wireless correspondent for the Daily Express.

The principle behind the circuit is very simple and almost self-explanatory.  The audio from the crystal set or other low-level source is fed to the traditional earphone.  A carbon button microphone is in physical contact with the earphone, and produces a stronger AF signal.  In this case, this stage is able to drive a loudspeaker.

This idea was rarely used in radio, since a vacuum tube amplifier provided better results and little additional cost.  The carbon button amplifier was more commonly used in telephone circuits, where they were the only method of amplification available prior to the vacuum tube.  They made long distance telephony possible.  They did have the advantage of a smaller size than a vacuum tube, and required less battery power.  Therefore, they did remain in use in hearing aids until the advent of the transistor three decades later.  You can read more about the carbon button amplifier at this site.

The advantage for the home constructor was probably cost, since driving a speaker this way would not require an expensive vacuum tube.  In fact, the carbon button amplifier could probably be constructed at home, which would be impossible in the case of a vacuum tube.  Students looking for a very novel science fair project might consider making one, since it would be possible to produce loudspeaker volume with entirely homemade components.

Highland Park College, Des Moines, IA


A hundred years ago, magazines devoted to electricity or mechanics were full of ads for learning radio.  A large number of these focused on training ship wireless operators.  There were other exceptions, but most such schools were located near the sea, in locations such as New York.

One of the exceptions that caught my eye was this ad for the “very thoro” wireless training program offered by Highland Park College in Des Moines, Iowa, shown here as it appeared in the March 1917 issue of Electrical Experimenter.

The ad promised the opportunity to see the world and draw a big salary as a wireless operator.  And the first stop in seeing the world was Des Moines.  This is actually not surprising, since the Hawkeye State was a hotbed of early wireless activity, with more than its fair share of amateur operators, and later, broadcast stations and companies involved in radio.

The college offering this course, Highland Park College, is no longer in existence, but had its own colorful history.  The school’s Wireless Building, apparently the location where students would be trained to see the world, is pictured here in the school’s 1914 yearbook.

According to the yearbook, the college had a wireless club which had been organized in April 1913, and had a complete sending and receiving set. The book boasted that the station’s large aerial allowed reception of the Arlington, VA, and Key West, FL, stations on a regular basis.


The college was established in 1889 and operated under that name until 1918.  It was apparently independent when founded, and in 1911 was transferred to the Presbyterian church.

1915MarPMWireless telegraphy was only one trade that could be acquired at Highland Park College, as shown by this ad in the March 1915 issue of Popular Mechanics.  The school also offered courses in machinist, automobile machinist, and chauffeur.  The machinist courses ran 48 weeks, whereas the chauffeur course of study could be completed in 12 weeks.  The school also offered a “special 6 weeks driving course.”

In 1918, the college was acquired by the Baptist church and renamed Des Moines University. Things went smoothly until 1927 when a fundamentalist wing known as the Baptist Bible Union of North America, the forerunner of the General Association of Regular Baptist Churches, took control. The faculty were required to subscribe to eighteen articles of faith. At that time, the University’s school of pharmacy was apparently the strongest department, and the faculty apparently had doctrinal differences to the point where they refused to sign the eighteen articles of faith. They departed and formed the Des Moines College of Pharmacy in downtown Des Moines. All but two of the pharmacy students left to enroll in the new school.

In addition to the doctrinal requirements imposed on the faculty, the students were facing restrictions. Three girls were disciplined for doing cartwheels during a vaudeville skit.

By 1929, the administration had enough, and fired the entire faculty. A riot broke out, and angry students stormed the administration building during a meeting of the board of trustees. Eggs and rocks were thrown, and the angry students attempted to break down the door of the room where the board members were hiding. Police were called, but the school formally closed down in September 1929.

The buildings sat vacant until 1943, when professional baseball player and aviation pioneer Alfred W. Lawson bought the property and founded the Des Moines University of Lawsonomy, Lawsonomy being billed as “the study of everything.” As might be expected for the study of everything, a degree was not something that could be earned quickly. According to Lawson, the students (men only) would need to study for 30 years to get their degree of Knowledgian.”

Enrollment peaked at a hundred students, but dwindled to 20 when the school closed in 1954 (presumably, with none of the students earning the coveted Knowledgian degree).

The property was sold, two weeks before Lawson’s death, and became the site of the Park Fair Shopping Mall.

Highland Park College in 1914. Library of Congress.

Highland Park College in 1914. Library of Congress.


The school’s location today. Google Maps.


1947 “Little Giant” All American Five


For many years, the March issue of Popular Mechanics had the tradition of carrying the plans for a radio receiver billed the “Little Giant.” A few weeks ago, we featured the 1942 version, and today we offer the version shown 70 years ago in the March 1947 issue.

1917MarPMLittleGiantSchematicThe circuit diagram for the 1947 version will look very familiar with those who’ve dug into postwar AM radios, since it’s the classic “All American Five” circuit employed for many years by most radio manufacturers. This one has the familiar complement of octal tubes: 12SA7GT, 12SK7GT, 12SQ7GT, 50L6GT, and 35Z5GT. This is also known as an “AC/DC” set, since it could run off either AC or DC 115 volt household current. It’s transformerless, since it rectifies the line cord for the B+ voltage, and wires all of the filaments in series to run directly off the line current.

This circuit does have one interesting postwar twist. Variable condensers were still in short supply, so it uses permeability tuning. Instead of a variable condenser and fixed coil, it uses a fixed condenser and variable coil. The inductance is varied by moving a plastic molded iron core in and out of the coils.

Old Glory: Banned in Boston


We recently carried an image , a smaller version of which is shown at the right above, of the SS Kansan, illustrating how the U.S. flag was illuminated to make abundantly clear that the ship was a neutral vessel.  The image appeared on the cover of the January 1917 issue of Electrical Experimenter.

Newsstand readers in Boston, however, didn’t see the flag.  Instead, some of them saw a sticker of Santa Claus.  Massachusetts law forbade the sale of goods displaying the flag, so news dealers were forced to obscure it.  In this case, Santa Claus got the honors of being the censor.  The image above is taken from the magazine’s March issue, which explained the odd juxtaposition.

1937 Armchair Radio


Eighty years ago this month, the March 1937 issue of Popular Science showed how to put together this receiver, which undoubtedly provided armchair copy of local broadcast stations. The author, Clark Maxwell, (not to be confused with James Clerk Maxwell of Maxwell’s Equations fame) explained that he had tried for several years to find a convenient but inconspicuous place for the radio in his living room. He decided to solve the problem once and for all by designing this set to hang on the arm of his favorite reading chair.

He billed the set as the “Arm-Chair Five,” due to the fact that it had five tubes inside. The radio itself consisted of three tubes, since one served as the rectifier. The fifth “tube” was actually a ballast to drop the voltage of the filament chain.

The circuit was a TRF, with one 6K7 serving as RF amplifier. A 6J7 and 25A6 served as audio amplifiers to drive a speaker.

The author used the set with a 25 foot indoor antenna, although he noted that an outdoor antenna could be used.

Since the chassis was “hot,” he stressed that under no circumstances should an external ground be used. The set was encased in a wooden box covered with fabric to match the chair.


Abdication of Czar Nicolas, 1917

Nicholas II, Tsar.jpg

Czar Nicolas II. Wikipedia photo.

On this day a hundred years ago, March 15, 1917, Czar Nicolas II of Russia abdicated the throne. Although his government continued for a time to tenuously hold power, he and his family were eventually executed in 1918.

1917 New York Police Wireless Telegraph


A century ago, cops knew Morse Code, as shown by these New York City police officers appearing in the March 1917 issue of Popular Mechanics.

According to the accompanying article, instruction in wireless telegraphy was being given to the officers, and a wireless apparatus had been set up at headquarters. That station worked in conjunction with other stations around the harbor, and also with a number of private wireless stations that had been placed at the service of the police for emergency work.

Motorcycle couriers would carry messages from the wireless stations to district and precinct offices. At press time, the department had eight certified wireless operators, and sixteen more were under instruction.

1957 British “Mini-Max” Transistor Portable


Sixty years ago, it was clear that the transistor was here to stay, and virtually all of the magazines for electronic hobbyists were filling up with construction articles featuring solid-state circuits. More often that not, the project was a transistor radio of some sort. This British example appeared in the March 1957 issue of Radio Constructor magazine.

This set was billed for local reception, but it made up for any lack of sensitivity in the fact that it would provide “loudspeaker” volume. In this case, the speaker was actually originally intended as a headphone element, but the set would put out 40 mW of audio, more than enough to listen to the set without any difficulty.

The set was constructed in a sandwich box cut to size. The author noted that the dimensions given in the construction article were intended for one particular brand of box. So if you want to recreate this set exactly, you’ll need to track down a British “Elevenses Pack” sandwich box. The circuit inside the sandwich box consisted of a diode detector followed by four stages of audio amplification. The set had a built-in loopstick antenna, and would presumably pull in those local stations without an external antenna or ground connection.

The author noted that the industry wasn’t quite ready to supply experimenters with components to construct a small superheterodyne receiver.  Small IF transformers were available, but none of them were in the subminiature category.  Transistors suitable for medium wave were hitting the market in the U.K., but the small transformers needed for a superhet weren’t yet available “comparable to those available in the States.”  But this crystal set with lots of audio amplification would serve the purpose for pulling in the strong local signals, and the hobbyists who built it were undoubtedly the first on their block to own a transistor radio.


Substitute Teaching in Minnesota

No, you don’t need a teaching license!

Among the many hats I wear, in addition to writing this blog, is that of substitute teacher. Depending on how busy my schedule is with other business, I teach in one of the Twin Cities area districts a few days a week.  For many people, this is an excellent part-time job opportunity to make a few extra dollars, make a difference in the lives of students, and possibly find yourself energized by exposure to their youthful exuberance.  The information on this page explains how to become a substitute teacher in Minnesota, although much of the information will be relevant in other states as well.

The rate of pay for this work varies from district to district, but in Minnesota, typically it is about $140 per day. Depending on the school, the substitute is expected to arrive about a half hour before the students arrive, and typically leaves about the same time as the students. This means that the substitute typically works about seven hours a day, and the afternoon is usually free.  The mathematically astute will realize that this is about $20 per hour, which isn’t too bad on days when I didn’t have anything else scheduled.  There are also frequently half-day assignments available. During the day, the substitute usually has at least one “prep hour” available. During the prep hour, the normal teacher catches up on other work such as preparing lesson plans or grading papers. Since the substitute usually isn’t expected to do such things, this usually results in another hour during the day to catch up on other work. Since there is almost always a telephone and computer with internet access available, this can usually prove to be a productive time.  Thus, even with the relatively low salary and lack of benefits, substitute teaching can be a reasonably lucrative proposition for many.

A person would certainly struggle to pay the bills if substitute teaching were their only source of income. But the schedule is so flexible that subbing can provide a great source of extra income for those who are self-employed or work another job in which they are free during the day.

For me, the main advantages of substitute teaching are:

  • A source of income on days when I’m not working elsewhere.
  • An opportunity to be an “insider” at my children’s own schools.
  • The opportunity to make a real, albeit brief, positive impact on students.

For those who do need the income, it would be possible to work almost every day as a substitute. Jobs are available on an almost daily basis. For those who do not need to work every day, this allows you to be very selective on which jobs you take.

Minnesota Requirements

In Minnesota, anyone with a four-year college degree in any subject can become a substitute teacher. You do not need to have a degree in education. This is because many school districts have a shortage of substitute teachers. In Minnesota, a person with a four-year degree, but without an education degree, and receive a “Two Year, Short-Call Substitute” license.

This certificate is issued by the Minnesota Department of Education. There is a fee of about $93, and you will need to be fingerprinted as part of the application process. As the name implies, this permit is valid for two school years.

Even though you will need to obtain this state license, the starting point is the individual school district where you plan to work. This is because the two-year license is available only to persons teaching in districts where the superintendent has verified that the district is experiencing a hardship in locating fully licensed teachers. As far as I can tell, the license, once issued, is valid statewide. But to get the license in the first place, you will need the signature of a district superintendent verifying that district’s hardship.

Fortunately for you, many districts in Minnesota are experiencing such hardships, and they will be overjoyed to sign off on your license application. In fact, they routinely do this as part of the hiring process.

I have noticed that these “hardships” seem to come and go. For example, the district where I am currently teaching does not currently have this hardship. Therefore, I would not be able to be hired there as a new substitute. However, once I’m in the system, I can keep teaching there. And since they frequently have substitute jobs that go unfilled, I wouldn’t be surprised if they once again declare a hardship and hire new substitutes such as me.

A quick Google search reveals that the following Minnesota school districts are currently hiring subsitutes and are willing to sign the certification so that you can get your license.  So if you live in or near one of these districts, they would be the ideal starting point.

Please note that these districts are the ones that currently publicize on their website that they’re willing to sign your application, and they are the ones I found with a quick Google search. There are undoubtedly many others.  (Some districts might not want to publicize on their website that they’re experiencing a shortage, so a phone call might be productive.)  To find these opportunities, check the district’s website, ask at your children’s school, or call the district. Many other Minnesota districts work with a firm called Teachers On Call, which handles the application process. Many of these districts will probably be willing to hire persons with a limited license as well.

The Hiring Process

When you inquire, you will probably be asked to apply in person. And it’s quite likely that you will be hired on the spot, subject to obtaining your license. You’ll probably walk out with the required form signed by the superintendent, who is happy to learn that his or her chronic substitute shortage is one step closer to being solved. Chances are, the staff at the district will be able to assist you with the process of applying for your license. When you visit the district office, you should plan on being hired that day. Therefore, it’s a good idea to bring along your college transcript, as well as the ID documents (driver’s license and passport or social security card) to complete all of the required forms that day.  (And don’t forget to bring your checkbook, since they’ll probably want a voided check to set you up for direct deposit.)  There’s generally no need to provide a resume, although if you have one prepared, it’s probably a good idea to bring a copy along.  When the license is approved, you’ll start getting jobs.

You will get little if any training. Most substitute teachers seem to be hired on a “sink or swim” basis. On your first day on the job, you will simply walk in, announce to the class that you’re their substitute for the day, and then make the best of the situation.  You’ll probably be given some kind of handbook or guide explaining some district policies, but you will be given little if any advice on actually teaching.

You will be told the mechanics of how you get jobs. Before the internet came into existence, school districts employed a person often known as the “gatekeeper.” This person would report to work at 5:00 AM and wait for teachers to call in sick. When they did, he or she would start calling substitutes to fill the vacancy.

The job function of the “gatekeeper” has now been largely automated. Instead of a human calling you, you will go to a website and/or receive an automated telephone call.  You will see all available positions and be able to select one.  My district, and most others, use a system called AESOP.  If you want to limit yourself to particular schools or grades, you have this option.  But your license allows you to teach any grade level from preschool through adult, and you’ll be given the opportunity to take any available assignment.  Since you don’t have to deal with a human being on the other end of the phone, it is very easy to be selective and take only the “good” jobs.

I currently have the telephone option turned off, and I get jobs strictly by logging in to the website.  I usually have these jobs lined up in advance.  If I were in need of daily work, I would set the alarm clock for 5:00 and wait for the phone to ring, safe in the knowledge that I would be working almost every day.

What the Work is Like

As a substitute teacher, there will be good days and bad days.  Fortunately, however, the good days far outnumber the bad.  And because you can pick your assignments, you never have to worry about going back to the bad classes!  After a while of taking jobs, you will recognize which are the good schools, which are the good teachers, and which are the good classes.  Armed with that knowledge, you can pick and choose and go back only to the good jobs.

Surprisingly, at first, it can be hard to predict which will be the good assignments and which will be the bad ones.  I’ve taught at schools with extremely bad reputations, and often found those assignments to be the most rewarding experiences.  On the other hand, I’ve also taken a few jobs at “good” schools where I don’t plan to go back.  For this reason, the early days of your substitute experience will teach you a few lessons.  But after you’ve figured out where the good jobs are, you’ll have days when you feel guilty about collecting a paycheck for such a fun assignment.

Because of my particular temperament, I  prefer taking jobs in high school, junior high, and occasionally the upper elementary grades.  I know that I wouldn’t be a particularly good kindergarten teacher, so I don’t take those jobs.  Other substitutes are more suited to younger kids and would be horrified at the prospect of teaching high school students.  The nice thing about subbing is that you can pick and choose.

One reason why I prefer high school and middle school is the fact that if I get a bad group of students, I know they’ll be gone in less than an hour.  I can put up with just about anything for an hour.  I rarely have miserable assignments, but it’s nice knowing that if I do, it’s of very short duration.

I have found that the principals, teachers, and all of the staff of the schools where I teach are genuinely happy to see me.  There is indeed a shortage of substitute teachers, and there are times when they need one but don’t get one.  When that happens, the other staff need to work harder.  The regular teachers often need to use their prep hour to cover another class, or the principal or another administrator needs to step in.  So when they see me, they’re happy to know that they don’t have to worry about the class that day.

Most times, the regular teacher leaves lesson plans.  This is often an activity where I need to do little more than hand out the assignment and sit back as the students do the work.  Many substitute teachers are happiest when they discover a lesson plan sitting on the desk.  On the other hand, I tend to enjoy the situations where there is no lesson plan and I’m left to fend for myself.  I consider myself a renaissance man, and I can always come up with something that ties in to what they’ve been studying, whether it’s math, English, social studies, science, or just about any other subject.  It’s my chance to pontificate, and yes, I enjoy showing off to the students that I can do the algebra problem and that, in fact, yes, we do algebra in the “real world” on a regular basis.

The most common question I’m asked about substitute teaching is how well the students behave.  Many adults recall their days as a student, and remember that when a substitute appeared in the room, the class erupted in chaos.

Thankfully, few days end like this. FEMA photo.

Thankfully, few days end like this. FEMA photo.

You will, indeed, experience chaotic situations from time to time as a substitute.  Some students will believe that they can get away with anything with the regular teacher gone, and they will try to do so.  However, by using a bit of common sense and displaying an aura of calm authority, most of these problems can be overcome quite easily.  Occasionally, it’s necessary to kick some student out of the room and refer him to the assistant principal or whoever deals with behavioral issues.  But this is actually quite rare, especially after students realize that you are willing to go to such extreme measures.  Typically, students will do their job with a minimum of prodding.

I’m also frequently asked whether substitutes need to understand the subject matter that they’re supposedly teaching.  The answer to this question is a resounding no.  The expectation is that the substitute will have absolutely no understanding of the subject matter.  If you maintain order for the day, you will be lauded for doing a great job.  You’re not actually expected to impart any knowledge to the students.

Having said that, my favorite part of the job is actually imparting knowledge, or at the very least showing off to the students that I actually understand the material.  So I take pride in explaining the causes of the Civil War on one day, and then applying the quadratic formula the next day.  The students are duly impressed, the teacher is pleasantly surprised to discover that the students actually learned something, and I’m probably requested the next time that teacher is absent.  But this is not the norm.  Normally, if the classroom is still standing at the end of the day, then you have done your job as a substitute, and everyone is happy.  So no, you do not need any particular knowledge of the subject matter in order to substitute teach.  Even if you have no idea what the quadratic formula is, you’ll still do fine teaching math classes.

Even in the worst classes (which are, thankfully, rare), it is clear that most of the students want to learn something.  It’s actually quite gratifying when a student thanks you at the end of class.  The rewards can come at unexpected times.  It’s not unusual to be teaching a history class and have a student ask if you can help with their math.  Occasionally, you’ll see that the student finally “gets it” after struggling with something for quite a while.  I’m not a better teacher than the regular teacher.  But I might bring a different approach that works better for one particular student.

If I haven’t scared you off so far, then I encourage you to become a substitute teacher.  The rules I’ve discussed here apply to Minnesota.  Most other states allow substitute teachers without an education degree, although the qualifications will vary considerably from state to state.  In fact, some require only a high school diploma.  (Pay in such states, however, seems to be considerably lower than states requiring a four-year degree.  One interesting possibility in such states is that college students can substitute while in college, although this is generally not possible in Minnesota.)  Each state will have a different set of hoops to jump through.  But most seem to have enough of a shortage of subs that they will assist you in every way possible as you jump through them.

You might get a small amount of training before your first job, but I’ve discovered that experience actually doing the job is much more valuable.  If you want to do some reading before undertaking the job, you might find some of the following books and websites helpful: