Monthly Archives: November 2020

Science Fair Idea: Jacob’s Ladder

This young woman undoubtedly took home the blue ribbon in the 1960 science fair. And she certainly earned herself considerable street cred as a mad scientist by putting together this Jacob’s ladder, by carefully following the plans in the November 1960 issue of Electronics Illustrated.

As revealed by the schematic diagram below, the circuit is simplicity itself. It simply takes normal household current and runs it into a transformer which steps it up to 12,000 volts, which is fed into two electrodes. One way or another, there is going to be an arc between the two conductors. It will follow the path of least resistance, and thus starts out at the bottom, where the two metal rods are closest together.

The arc heats the air right above it, which ionizes the air. The ionized air thus becomes the new path of least resistance, and the arc moves upward. The process repeats until it reaches the top, at which point a new arc forms at the bottom.

The critical component, of course, is the high voltage transformer. In 1960, it was available in the form of a neon light transformer, for $12 plus shipping. Of course, neon signs still exist, and you can buy the transformers on Amazon. This modern replacement is rated at “only” 10,000 volts, but it seems like that should be sufficient.

The 1960 article includes a number of important safety features. First of all, the electrodes are safely behind a sheet of 1/32″ clear acetate. This also appears to be available on Amazon.

And the choice of switch ensures that the device isn’t turned on accidentally. The power switch is a momentary switch, meaning that when you let go of the switch, it turns off. It’s a SPDT switch, meaning that when it’s off, another contact is energized. This is wired to a green pilot light that indicates that the device is plugged in. The switch has a safety cover, meaning that one needs to consciously lift the cover to flip the switch.

The article begins by saying that “if the directions given in this article are followed to the letter, there is no danger of shock. Don’t take short-cuts or omit the built-in safety features of this model; it is poor economy to leave out relatively inexpensive components that might prevent a nasty shock.”

These days, when one reads safety warnings, there’s a tendency to ignore them. But in 1960, when a warning like this was included, it’s because the activity in question was, indeed, dangerous, and you really needed to be careful. So I believe this project is suitable for mature students, working under the supervision of a competent adult. But this contraption is potentially lethal, and great care must be taken in its construction and use. In 2016, a Michigan teen was killed trying to build one, and we don’t want any of our readers to suffer the same fate. So please be careful.

1940 Clock Radio

The humble clock radio hit the scene shortly after the war, but as we can see from this self-explanatory drawing in the November 1940 issue of Popular Mechanics, one John L. Story of Washington, D.C., had come up with the idea. He mounted a small toggle switch on the back of a wind-up alarm clock not unlike the one shown at left, still available on Amazon.

When this type of clock rings, the key for winding the clock unwinds. The switch is carefully placed so that the unwinding key trips it, turning on the radio or other small appliance. For students interested in the history of technology, this would be an interesting science fair project to duplicate Story’s invention and answer the question, “what is the simplest clock radio?”

1920 Voting Machine

A hundred years ago, on November 2, 1920, this gentleman was casting his vote, for either Warren G. Harding or James M. Cox in the 1920 Presidential Election. (I suppose he might have been voting for Eugene Victor Debs, but he doesn’t look like a socialist.)

Whoever he voted for, he cast his vote on a lever-type voting machine, probably manufactured by the U.S. Standard Voting Machine Company. These were commonly in use for many decades thereafter, and when I first voted in 1979, it was in a machine that looked almost like this one. The voter pulled a lever to close the curtain, which unlocked the voting levers. When the voter is finished he (or she, since this was the first election after the ratification of the 19th Amendment) opened the curtain, which caused a counter at the back of the machine to increment for the candidate selected. When the polls closed, the election official unlocked the back of the machine (which locked the levers on the other side) and looked at the count for each candidate.

The picture appeared on the cover of Popular Science, November 1920, and the accompanying article proclaimed that the machines would insure an honest election. The article detailed all of the nefarious things someone could do with paper ballots, and proclaimed that “the machine is honest, and its honesty is fully protected from those who would destroy it.”

1940 One Tube Broadcast Receiver

This young man is pulling in a local station with loudspeaker volume using the circuit shown 80 years ago this month in the November 1940 issue of Popular Science. The set uses a 1E7G dual pentode. One half is an RF amplifier, and the other half serves as the detector, but with enough output to drive a speaker with no further amplification. For stations within ten miles, and indoor antenna can be used, and for local stations further away, a good outdoor antenna is necessary. Selectivity was said to be very good, with the circuit able to separate strong stations only 30 kHz apart.

The set is a TRF, meaning that each stage had its own tuning dial. Two ganged variable capacitors could be used, but the separate dials allow each stage to be tweaked for maximum sensitivity. The 90 volt plate voltage was supplied by two 45 volt B batteries, with a 3 volt A battery lighting up the filaments.