1925 Emergency Foot Bridge

1925FebSciInvBridgeA hundred years ago this month, the February 1925 issue of Science and Invention gave these self-explanatory plans for constructing an emergency foot bridge. It had the advantage of being able to use it as you constructed it, since you pushed each new section into place.

Each support was a pair of jumping poles. You would form them into an “X” shape, and then push them out ahead. Perhaps some student will be inspired to turn this into a science fair project. Or perhaps a scout looking for a good pioneering project might be inspired. (If the water below is deeper than ankle-deep, then the current Guide to Safe Scouting calls for participants to wear a life jacket.)



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Laser Safety: 1970

1970FebPEThe December 1969 issue of Popular Electronics carried the plans for a 0.5 milliwatt laser. The magazine noted that there was a lot of interest in making such a device, and there were finally parts on the market to make one that met two important criteria: It had to be inexpensive enough for experimenter hobbyists, and it had to be safe. They deemed the 0.5 milliwatt device to be safe, given some precautions, and the whole project could be built for under $50.

The illustration above appeared in the February 1970 issue, and it depicted how the magazine viewed a few of its readers. In summary, they wanted to burn holes in things, and they wrote in to the magazine asking for plans to do that. In response, the editors made clear that they had no plans to do such a thing. To cause destruction to something, the laser would require a few watts. But eye damage could easily take place at just a few milliwatts. The 0.5 milliwatt laser was safe, but more powerful units were not, so the magazine didn’t want to be involved. “Out of responsibility and concern for our readers, POPULAR ELECTRONICS cannot suggest or recommend a laser of any higher strength than the one described in the article.”

For the young mad scientists, it’s probably possible to find dangerous lasers on eBay, but we don’t plan to help, either.  On the other hand, you can get safe low-powered laser pointers on Amazon.



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How to Make a Pressman’s Cap

Screenshot 2025-01-30 11.16.41 AMWe know our readers pretty well, and we’re almost certain that when they watch an old movie showing a newspaper printing plant, they notice that the pressmen are wearing hats made out of folded newspaper. They also wonder how to make one.

If you fit in that category, you are now in luck, because you have access to the February 1940 issue of Popular Mechanics, which reveals on page 283 exactly how it’s done. It was sent in to the magazine by one W.R. Greer of St. Paul, MN, whom we hope didn’t get in trouble for revealing this trade secret.

For the sake of categories, we decided to place this one in “science fair ideas.”  For a last-minute project, this one answers the scientific question of, “can durable head wear be constructed from newspaper.”  For similar ideas, we’ve previously shown how paper can be used to construct boxes, and even expedient winter footwear.



1945: Keeping Your Name Before Customers

1945FebRadioRetailingEighty years ago there was still a war going on. But people knew that it wouldn’t last forever, and there was a pent-up demand for consumer products like radios that had been unavailable for the duration.

The cover of the February 1945 issue of Radio Retailing reminded dealers of the importance of keeping their names in front of consumers. When the war ended, and products were available, you wanted them to come to you to buy them.



1940 Homemade Batteries

Eighty-five years ago, Britain was at war, and that meant shortages of many things, including flashlight batteries. Undaunted, many Britons took to making their own, and the February 1940 issue of Practical Mechanics showed them how to do it.

Screenshot 2025-01-23 9.14.31 AMThere was a learning curve involved, but the magazine assured readers that the task was well within the capabilities of amateurs. The costs of materials were low, and once you were set up, you could laugh at the Nazis trying to deprive you of batteries.

The article pointed out that filling the cells, at least initially, was a messy process. But once you got into a routine, it was relatively easy. The article suggested getting together a quantity of zinc containers and carbon rods, and then commencing the filling process. Surrounding the carbon rod was a “depolarizing paste” consisting of a mixture of approximately equal quantities of carbon or plumbago powder and manganese dioxide (pyrolusite) made into a paste with a 1 per cent. solution of gum tragacanth. The electrolyte consisted of about 85 per cent. of plaster of Paris and 15 per cent. of ordinary flour mixed to a just-wet paste with a strong solution of sal ammoniac,

For the student looking for a science fair project, making a battery is always a worthwhile option.  In addition to this set of instructions, we have many other  similar ideas on this site.



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Admiral Model 12-B5, 1940

1940FebRadioRetailingEighty-five years ago, the War was still over a year away for America, but it was raging in Europe. With a shortwave receiver, you could pull in the war news directly from Europe. And thanks to the Admiral model 12-B5, you could do so for the unheard of price of only $12.95. Of course, there’s been a lot of inflation since 1940, and according to this inflation calculator, that works out to $290 in 2025 dollars. But we think it would be worth the investment.

This ad appeared in the February 1940 issue of Radio Retailing.



1950 Look at the H-Bomb

1950July30LifeThe first hydrogen bomb wasn’t tested until 1952, but two years earlier, readers of Life Magazine, January 30, 1950, got a pretty good idea of what it would be capable of. The above picture shows the blast radius for the A-bomb (small circle) compared to that of the H-bomb (large circle).

Colorado Senator Edwin C. Johnson had referenced the new bomb in a television broadcast, the press followed up, and President Truman had failed to either confirm or deny that the project was underway.  The magazine noted that the nation was still trying to adjust itself to life with the A-bomb, but the new bomb, which would harness the same energy as the sun itself, would make the Hiroshima weapon look like a stick of dynamite.



Ohio Girl Builds Radio, 1925

1925Jan29IndyTimesOne hundred years ago today, 13-year-old Grace Weaver of Oakwood Village, Ohio, near Dayton, made the front pages by putting together her own five-tube neutrodyne radio. On her first night, she pulled in Los Angeles.

She is shown here on the front page of the Radio section of the Indianapolis Times, January 29, 1925.



1965 Grid Dip Oscillator

1965Jan73GDO

Sixty years ago, the January 1965 issue of 73 Magazine showed how to make this simple Grid Dip Oscillator (GDO or “Grid Dipper”.)

The GDO is handy for quickly measuring the resonant frequency of a tuned circuit or of an antenna. The GDO’s coil is exposed, and it’s poked into the coil of the circuit in question. For an antenna, a small loop of wire attached to the antenna goes around the coil. The frequency is adjusted, and at the resonant frequency, the meter showing the grid current drops precipitously. If you need the exact frequency, you can find the signal of the GDO on a receiver, or use a frequency counter.

This example uses a 3S4 tube, and has the convenience of being able to run from batteries. Of course, these days, the same thing can be done with transistors. Even though transistors have no grid, many old timers still refer to the instrument as a Grid Dipper. Hams these days are more likely to use an antenna analyzer, but the Grid Dipper gets the job done in an elegant analog fashion.

1965Jan73GDO2



1945 Four-Tube Portable Station

1945JanQST11945JanQSTEighty years ago, there was still a war going on, and hams were off the air for the duration. But more than a few of them were itching to get back on the air as soon as possible, and one of them was apparently Sheldon W. Gates, W8VWK, who designed this portable station, described in the January 1945 issue of QST.

The three-band (80, 40, and 20 meters) station was housed in a small vanity case which measured 11.5 by 6.5 by 7 inches, and which set him back $2.49. He originally considered designing the set for battery operation, but since he described himself as a city boy who rarely went farther than the power lines, he settled on AC power. The transmitter used a single 117N7 tube, and the receiver was a three-tube Meissner Student Midget Kit that he cut down to fit the available space. Since the receiver used 1 volt tubes, he included a dry cell to run those filaments.

The cover of the case included plenty of room for logbook, key, crystals, and headphones. At the end of the article, he reminded readers that the transmitter could not be operated on the air at the present time.