# How does the blue razor blade work to tune this Superboy Radio? **URL:** \ **Category:** Factual Questions\ **Created:** [April 29, 2015, 6:24am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784 "2015-04-29T06:24:34Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [April 29, 2015, 6:24am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/1 "2015-04-29T06:24:34Z")
[Picture here](http://www.usmilitariaforum.com/uploads//monthly_06_2014/post-22-0-55438500-1401743744.jpg) How exactly is the “blue” razor blade acting as a tuner in this scenario? ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [April 29, 2015, 8:10am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/2 "2015-04-29T08:10:12Z")
The coated blade/pencil lead form a point-contact diode - basically a semiconducting rectifier. Finding the best point of rectification is all the tuning that happens. Without a variable capacitor, the resonance of the aerial and coil is the only real tuning going on. These sorts of crystal radios only really isolate the strongest signal, unless you have coil taps or a variable length antenna. The design dates back to [WW2](http://en.wikipedia.org/wiki/Crystal_radio#1940s). [QUOTE=wikipedia] In some circuits a capacitor is not used, as the antenna also serves as the capacitor. [/QUOTE] ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [April 29, 2015, 9:18am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/3 "2015-04-29T09:18:06Z")
Wow! That takes me back. We made these at school when I was a boy in the late 40s so we could listen to Radio Luxembourg on 208 meters. 11.00 PM Sunday, long after lights out, huddled under the blankets, listening to the Top Twenty – introduced by Pete Murray. [http://radiosoundsfamiliar.com/radio-luxembourg.php](http://radiosoundsfamiliar.com/radio-luxembourg.php) ---
**Author:** ![Nefario](https://avatars.discourse-cdn.com/v4/letter/n/59ef9b/32.png) [@Nefario](https://boards.straightdope.com/u/Nefario)\ **Post date:** [April 29, 2015, 10:59am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/4 "2015-04-29T10:59:48Z")
I believe that people who build crystal radios today usually have a good commercial diode handy to get it tuned in. Then they try to find a signal using the razor blade or a natural crystal. The blued surface and pin point act as a diode; I’ve read that some blue blades work OK, others not at all. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 29, 2015, 12:01pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/5 "2015-04-29T12:01:22Z")
> [@si\_blakely](#): > > The coated blade/pencil lead form a point-contact diode - basically a semiconducting rectifier. See [Cat’s whisker detector.](http://en.wikipedia.org/wiki/Cat%27s-whisker_detector) ---
**Author:** ![Tom\_Tildrum](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@Tom\_Tildrum](https://boards.straightdope.com/u/Tom_Tildrum)\ **Post date:** [April 29, 2015, 3:13pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/6 "2015-04-29T15:13:13Z")
I love how the design directs kids to work with a razor blade, without even the slightest warning or suggestion of parental supervision. The past is another country. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [April 29, 2015, 3:55pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/7 "2015-04-29T15:55:50Z")
the blade is a detector. it would be the same as a crystal in a crystal radio. there are many dozens of circuits these so not all schemes seem to have all components but they are there. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 29, 2015, 5:26pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/8 "2015-04-29T17:26:30Z")
One thing that’s important - a “blued” razor blade (or sometimes a rusty one) is required. The corrosion or oxidation on the surface of the metal is necessary to make the diode. ---
**Author:** ![Omar\_Little](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omar_little/32/269_2.png) [@Omar\_Little](https://boards.straightdope.com/u/Omar_Little)\ **Post date:** [April 30, 2015, 7:26pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/9 "2015-04-30T19:26:22Z")
This a version of a foxhole radio that were commonly made by GI’s during WWII. Here’s a youtube video of how to make one: [How to Make a Foxhole Radio - YouTube](https://www.youtube.com/watch?v=skKmwT0EccE) ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [April 30, 2015, 7:51pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/10 "2015-04-30T19:51:36Z")
Do they still even make that kind of razor blade? I remember them, but I don’t recall seeing any recently, with all the double, triple and quad blades that the razor companies sell now. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [April 30, 2015, 8:08pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/11 "2015-04-30T20:08:43Z")
single edged paint scraping blades exist and though not blued a bit of rust might make it work. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [April 30, 2015, 8:13pm UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/12 "2015-04-30T20:13:34Z")
Ah, all the better: “Kids, if you can’t find a blue one, you can make do with a nice rusty razor blade instead.” ---
**Author:** ![busterpickle](https://avatars.discourse-cdn.com/v4/letter/b/258eb7/32.png) [@busterpickle](https://boards.straightdope.com/u/busterpickle)\ **Post date:** [May 1, 2015, 4:00am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/13 "2015-05-01T04:00:27Z")
What was the blue coating? Probably to provide some grip or corrosion protection while clamped in the razor. However, I question it’s real purpose as in the OP’s link they suggest to scratch the surface to get a better signal. Quite possibly, they are referencing a “blue razor blade” as a style of blade to get. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 1, 2015, 4:04am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/14 "2015-05-01T04:04:28Z")
> [@electronbee](#): > > What was the blue coating? Probably to provide some grip or corrosion protection while clamped in the razor. > > However, I question it’s real purpose as in the OP’s link they suggest to scratch the surface to get a better signal. Quite possibly, they are referencing a “blue razor blade” as a style of blade to get. It’s “[bluing](http://en.wikipedia.org/wiki/Bluing_(steel)).” The oxidation is necessary to create the diode - you can’t do this with a new, clean razor blade. ETA - From the link above: > [@](#): > > Historically, razor blades were often blued steel. A non-linear resistance property of the blued steel of razor blades, foreshadowing the same property that would later be discovered in semiconductor diode junctions, along with the ready availability of blued steel razor blades, led to the use of razor blades as a detector in the crystal set AM radios which were often built by soldiers during World War II.[2] ---
**Author:** ![Omar\_Little](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omar_little/32/269_2.png) [@Omar\_Little](https://boards.straightdope.com/u/Omar_Little)\ **Post date:** [May 1, 2015, 6:15am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/15 "2015-05-01T06:15:59Z")
> [@beowulff](#): > > It’s “[bluing](http://en.wikipedia.org/wiki/Bluing_(steel)).” > The oxidation is necessary to create the diode - you can’t do this with a new, clean razor blade. But according to the youtube I linked above, you can accomplish the same thing on new blade by heating it up. See the 2:01 mark on the video. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [May 1, 2015, 10:03am UTC](https://boards.straightdope.com/t/how-does-the-blue-razor-blade-work-to-tune-this-superboy-radio/718784/16 "2015-05-01T10:03:51Z")
Heating the blade allows some surface oxidation to occur, causing the bluing.