Excellent answers so far.

I would attempt to connect the dots between recording and red button like this:

Apart from the traditional warning usage, a red light has been used in many scenarios to represent on-going work - radio show room when on air, operation theater when operation is going on, etc. The reason behind that would be the same - it is highly noticeable in the human visible color field. It acts as a signal to someone who might not be aware of what is going on, to prevent them from interrupting the process. This is highly desirable when you are recording something. And it can be easily used in social context: Rather than 'telling' someone that you are recording and they should not make a sound, you can just point to the red dot on your microphone/recorder and they would understand the situation. I have even seen some microphones with a small red indicator on when the device is on.

As far as the button's shape goes, I do not think it has that much history in it apart from a couple trend setters. But, a good explanation would be: All the other controls on a radio/cassette player were rectangular in shape. The record button demanded some extra affordance so that it was not accidentally pressed (overwriting something which you wanted to hear!). A couple options are: give it a different shape - circular button - and highlight it - red color. In many cases you would see just the red color being used (or even just the red symbol on a regular button).

The symbol on each button was created with references to sheet music ^[1] and inventor's background.

• For example, the || in pause may come from the Japanese character リ and/or Caesura ^[2].

• The media control UI were first introduced by Swedish Engineer Philip Olsson while he was working in Japan. He also had a degree from Swedish design school.

The glyphs were standardized by International Electrotechnical Commission ^[3] in Geneva under IEC417 Graphical symbols for use on equipment

In early 20th century - Record was denoted by 3 wave lines in a ReVox.

Today's standard Media Control convention as seen on a the Panasonic RX-5150 boombox (which is 2 decades old)

The record button is round because of the triangle the square and the rectangles being already chosen for the play, rewind, forward, stop and pause buttons, and the need for an eye-catcher for security reasons, that is preventing critical information erasing.

When is applied, the red color is applied for the same reason.

I suspect it follows the development of machines to play compact cassettes (which were developed by Phillips).

Wikipedia says ^[1] that this was the first player that Phillips released to play the tapes

Philips also released the Norelco Carry-Corder 150 recorder/player in the U.S. in November 1964.

Here it is: and it has a round red record button.

http://www.shizaudio.ru/audio/data/media/32/86866a_2.jpg

The red colour is logically a warning colour - as pressing record overwrites what you've already got on the tape !

I assume it comes from the "ON AIR" broadcasting signs, which would traditionally have a red light to indicate that the studio was recording.

I assume that other recording items then had a red light, since the "ON AIR" sign would be overkill for a small device.

Eventually, the idea of a light to indicate recording would be too expensive, especially in an era before LEDs, and so the red light was replaced with an iconographic representation - a red circle.

Source: Totally making this up as I go along.

I can partly answer about the colour, but not about the round shape. Colour has culture-specific meaning ^[1], and red varies from happiness (in China) to death (in Egypt).

I assume that most European, American, and Japanese manufacturers used and standardised red according to their notion of red meaning danger. Also, because their customers would probably make this association, as these were the profitable markets when recorders were first made, unlike nowadays ^[2].

The usage certainly pre-dates cassette tape recorders. Dieter Rams reel-to-reel tape recorder the TH 60 of 1965 had a red circle on the record button.

http://www.formguide.de/en/designers/overview/dieter-rams/

I don't know if it was established before then.

A possibility is that in the days preceding tape recorders, there were wire recorders. And as a primitive equivalent of the VU-Meter, they used a neon bulb, which would flicker when the recording level was right. Too low and the neon wouldn't come on. Too high, and it would be solidly on. And Neon bulbs were red. So a red circle could have become associated with recording that way.

is it not also a great example of consistent UI across physical products, a level of consistency that those manufacturing computer software, hardware and designing for the web could actually learn a lot from.

i.e. once a few products set the standard is has then been adhered too across pretty much all products so that the user knows what will happen when they press the round, red button

Another reason could be that red color is easy to spot. You can distinguish red from a crowd of other colors easily. Same case with green. Our eyes are more sensitive to green than any other color and that is why greenery is soothing to our eyes. So all the important buttons come in either green or red and the button with which we should be more cautious with comes in the red color.