EEs: If On Gilligan's Isle...

If a motor can be a generator if run backwards, can a radio be rewired to be a transmitter? - Jinx

No. Well, actually a radio receiver of the most common type, does transmit a weak signal but not on purpose. In fact, the amount of signal leakage from a receiver is regulated by FCC emission standards. The explanation for this is a little long so just take my word for it this time, OK?

And an electric motor doesn’t have to be run backwards to be a generator.

Electric motors don’t have to run backward to be a generator? Ok, explain that one to a Mech. Eng.! - Jinx

The dc through the field coils establishes a magnetic field around the rotor. The rotor contains conductors. When a conductor moves through a magnetic field a voltage is generated in the conductor irrespective of the direction of motion. All changing the direction of rotation does is reverse the polarity of the generated voltage.

The induced voltage e[sub]i[/sub] = k*d[symbol]F[/symbol]/dt, [symbol]F[/symbol] is the magnetic flux linking the conductor and k is a constant whose value depends on the units in use. This has a value regardles of the sign of the derivative.

Of course, if you induce a magnetic flux while driving at 88 miles per hour, you can introduce your parents to each other.

You can make a transmitter out of a radio very easily, in fact.

As far as Gilligan’s Island, they would have been better off making a “spark gap” transmitter, these were used in the earliest days of wireless and illegal since the 1920’s or so, as they have a very wide spectrum and pretty much step on everything. I imagine a generator, batteries and an ignition coil and such, and the professor would have been able to tap out an SOS via morse code.

Basically, an old tube radio (an"AA5") is the easiest to modify, the output tube which drives the speaker is rewired so it plate modulates the converter stage. Lots of hobbyists rewire sets so they can listen to tapes or CD’s around the house over their other radios or stereos. AES, sells a ‘phono oscillator’ transmitter utilizing 12SA7, Radio Shack, Vectronics, and others sell solid state versions.

Be carefull – signal propagation is in part determined by the length of the antenna and you don’t want the FCC pounding on your door…

I always wondered… how did the Professor know that the transmitter was working? When stuck on a deserted island, obviously the first thing you need to do is to build is a bunch of test equipment!

Here’s a pretty cool transmitter. Basically you rewire a crystal radio to become a signal-emitter:

Now if it was 1930, then “regenerative” radio receivers were pretty common, and “superheterodyne” types hadn’t yet taken over. It’s easy to rewire a “Regen” to become a transmitter. (As with motor/generators, a “Regen” radio is already a transmitter, that’s why it works so well. See,

If I recall correctly the Professor asked the Skipper, “Can you modify this radio to be a transmitter?” The Skipper replied, “No problem, Professor! All I have to do is reverse the transistors. We used to do it all the time in the Navy.”

So while I believe it is possible to utilize the components from a radio to build a transmitter, it’s not a matter of “reversing the transistors.”

The Professor would have been able to make at least an AM/SW receiver with the materials at hand – presuming they had some way of making a high impedance headset – a detector can be made with nothing more than a razor blade as a diode and a pencil lead for point contact – and of course a ground wire and long antenna aka “World War II Foxhole Radio”;

AES also sells these for $10.95, though most people can find their own razor blade and pencil lead themselves…

While we’re asking scientific questions: can being hit on the head with a coconut really make you think you’re Ginger Grant?

I knew that.

I’m sure the Professor could figure out a way – But you know as soon as it was done Gilligan would knock it into the lagoon-- rendering it useless and earning him a whack on the head from the Skipper’s hat. That’s good comedy.