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?”
We previously featured the idea shown at left, using a dual-filament light bulb as a diode tube. The idea is to burn out one of the filaments, and use that as the plate. The other filament becomes a directly heated cathode. This is from the January 1943 issue of Radio News, which also included other ideas for emergency crystal sets. This one called for a 6-volt type 1158 lamp, which is still available on Amazon
The same idea appeared a hundred years ago this month in the September 1920 issue of Popular Science. This one was to use the bulb as a rectifier and not detector, but the idea is the same. The idea was sent to the magazine by one R.U. Clark, 3d:
The bulbs used as rectifiers are the bayonet-base type, round automobile headlights of the double-filament type, just recently put on the market for the Ford automobile. One filament should be burned out by an over-voltage current applied to the proper terminals. The connecting wire and all that remains of this filament can then be used as the plate, and the remaining filament, which consumes only .85 amperes, can be used to supply the electron stream.
The usual alternating current can be fed to the filament and plates of these bulbs when suitably reduced by transformers, and direct current taken out in the manner usual with such devices.
These tubes will pass about 0.5 amperes each under suitable conditions.
The author notes than many want to experiment with vacuum tubes as rectifiers, but don’t want to run the risk of damaging expensive tubes. The Ford light bulbs, on the other hand, sold for only 65 cents, making such experiments more forgiving. The illustration here shows to bulbs, presumably mounted as a full-wave rectifier.
Making a homemade vacuum tube in this manner would make a very interesting science fair project. To demonstrate, an old “wall wart” transformer could be used as the input. Most have a DC output, but some have a low voltage AC output. Examine the ones available at the closest thrift store, and chances are, a suitable one can be found. By using an inexpensive multitester that measures both AC and DC voltage, it can be shown that the input is AC and the output is DC.
If Junior is looking for a spectacular science fair project that can be built at little expense, but still uses a slightly dangerous chemical, he can’t go wrong building this real submarine, according to plans contained in the August 1940 issue of Popular Science.
School will eventually reopen, so your young submariners can take advantage of their summer vacation to build and test the craft, which can be made out of scraps of sheet metal and tin cans. Soldering is required, but kids naturally enjoy working with molten lead. When school reopens and the science fair is on, the submarine can be used for a variety of scientific experiments involving buoyancy, propulsion, or chemical reactions. The teacher has probably seen old movies with miners wearing headlamps, but this will probably be their first exposure to the actual chemical used. A blue ribbon is almost guaranteed.
The young man shown here is probably getting ready to collect his first Social Security check, but fifty years ago, he undoubtedly took home the blue ribbon at the 1970 Science Fair. His project was shown in the August-September 1970 issue of Science and Electronics. According to the magazine, the project would show off knowledge and understanding, but would also require dexterity with tools.
The project was a moving-coil ammeter, not unlike a commercially made meter movement. While the magazine didn’t use the moniker, this type of meter is also commonly known as a d’Arsonval meter movement, after Jacques-Arsène d’Arsonval. For less advanced students, the article referred back to construction articles for some more primitive meters, such as the hot-wire ammeter we previously profiled. Most of the meter’s mechanical parts were made of wood, so the project required some expertise in the wood shop. The form for the moving coil was made with balsa wood, whose light weight ensured a sensitive movement.
The finished product could be accurately calibrated by using a commercial VOM, a battery, and a potentiometer. Current readings were noted on the commercial meter, and then marked on the face of the homemade meter.
According to the article, if the builder used reasonable care and followed instructions, they would be assured of a good grade and congratulations from friends and teachers. And fifty years later, there’s no reason to think the result would be any different.
I’m not sure it would comply with modern OSHA rules, so whatever you do, make sure you don’t touch any of the exposed conductors. But if you need to quickly come up with a low voltage, this self-explanatory method of making your own resistor should do the trick. Measure the voltage, and keep adding salt until you get the desired voltage.
The idea appeared in the August 1945 issue of Radio Craft. It had been sent in to the magazine by one Ollie Peoples of Mountain View, Oklahoma, who pointed out that if you can’t reach the desired voltage, then use a larger glass and add more salt. He reported that the average glass wouldn’t deliver more than six volts, and would get unduly hot with that voltage.
As long as they’re careful with the high voltages involved, advanced students could use this idea as part of an interesting science fair project, demonstrating how changes in salinity affect the resistance.
For the young scientist who wants to outsmart the science teacher, here’s an excellent science fair project that duplicates the work of American physicist Henry Rowland.
Your teacher undoubtedly knows that a an electrical current generates a magnetic field. But what your teacher probably doesn’t know is that a moving charge of static electricity also generates a magnetic field. This can be demonstrated from this experiment in the July 1940 issue of Popular Science.
To do the experiment, you set up an electric motor as shown here. You attach a disk of hard rubber, which you electrify by rubbing it with a woolen cloth. (Instead of the rubber disk, you can use an old phonograph record. If you don’t know what that is, you can ask your grandparents, or read some of our posts about the history of the phonograph).
Once the disk is charged up, you turn on the motor. As soon as it starts spinning, a compass placed nearby will deflect, showing the presence of the magnetic field.
If Junior decides to do this science fair project from the the July 1940 issue of Popular Science, he should probably change the name. The magazine calls this gadget a carbon dioxide “gun,” and he’ll probably get in a lot of trouble if he calls it a gun. If he calls it a “remote fire extinguisher,” he’ll probably get a blue ribbon instead of a visit to the police station.
Like all young men like to do, the two shown here are playing with fire, but making a scientific point in the process. In this William Tell stunt, one young man has a candle on top of his head. The other one puts it out with the Carbon Dioxide Gun–er, I mean remote fire extinguisher. Whatever it’s called, the device is simply a can with a balloon stretched over one end. Inside, a piece of dry ice is placed. When the balloon drum is tapped, an invisible cloud of CO2 is expelled, which causes the flame to be deprived of oxygen.
I suspect many of our readers have independently invented the idea shown here for an easy way to cook a hot dog. You simply run 120 volts through the hot dog, and the hot dog serves as a resistor and cooks itself. This incarnation of the idea was designed by prolific electronics writer Len Buckwalter and appeared in the July 1960 issue of Electronics Illustrated.
This design is a bit safer than what I remember doing. While cooking, the hot dogs are safely concealed inside a bakelite box. They don’t start cooking until the lid is inserted, since the cord is part of the lid, and there’s a TV-style safety interlock.
The young scientist wishing to come up with an interesting science fair project won’t go wrong with this idea. It give a great demonstration of Ohm’s law and the power law. And sharing the hot dogs with the judges certainly won’t hurt in earning that coveted blue ribbon.
Of course, you don’t want to be disqualified by electrocuting one of the judges, so it’s best to come up with some form of interlock.
The venerable Presto Hot Dogger used the same principle to cook hot dogs. It unfortunately seems to be out of production, but they show up on eBay. But making your own is easy, and a lot more fun.
If you’re looking for a science fair project that can be put together with parts you’ll find around the house, you can’t go wrong with this motor from the July 1940 issue of Popular Science.
The main components used are paper clips. You’ll also need some insulated copper wire and a few other odds and ends. The old-fashioned dry cell batteries look cool, but it will work just as well with a couple of alkaline D cells.
Fifty years ago this month, the June-July 1970 issue of Science and Electronics showed how to put together this electrostatic generator, as well as the accompanying Leyden jar and electroscope.
It consisted of a plastic jar which was turned by the crank to rub against a piece of wool. A pickup wire collected the charges and conducted them to the metal ball. This could be used to charge the Leyden jar. For bearings, the device used roller skate wheels.
Emergency preparedness buffs will notice that the electroscope, at left, looks very similar to the venerable Kearney Fallout Meter (KFM) used for detecting nuclear radiation. Indeed, they do rely on the same principle. In the presence of ionizing radiation, the charge will slowly dissipate, allowing the leaves to get closer together. This electroscope is not calibrated. Therefore, it would not be able to provide an accurate radiation dose reading. However, this generator would be useful to charge the KFM.
For young scientists without any ionizing radiation at their disposal, the magazine describes a number of interesting experiments, any one of which would be sure to bring home the blue ribbon at the next science fair. To guarantee the top prize, one interesting experiment would be to purchase a small uranium sample. Charge up the electroscope and see how long it takes to discharge. Then, repeat the experiment with the uranium inside the electroscope and see how much faster it discharges.
