1895 Headphone Jack

With all the talk of HDMI, Lightning, and USB-C, we often forget about the one, ubiquitous connector that sits beside them all: The humble TRS headphone jack, (aka the phone connector)

The first evidence we can find of the cord is this patent filing for a "plug and spring jack for telephone boards" from 1895 (pictured right) that seems to correspond with the modern TRS. "The invention is applied particularly to a 'triple plug' and a corresponding 'three part spring jack,'" explains the patent, leading us to believe this is the same invention. The TRS name stands for the three rings, which make up a triple plug. Also, this Electrical Review article from 1903 says these triple plugs were used for telephone jacks during that time. The evidence adds up. 

As you can see, that patent drawing looks a lot like what we see today -- it almost looks like a hip throwback advertisement for an auxiliary cable. The final conception back then, which we see pictured below via International library of technology, didn't differ much in figure from either the older patent drawing, or the modern plugs we see today. For over 100 years, this plug has looked a lot like that. 

Though these originated in the telephone world, today we use them for all sorts of audio transmission, like headphones. But, even so, the basic use is the same as it was 100 years ago: To move sound. 

The connectors come in 3 sizes: 1/4" or 6.35mm, 1/8" or 3.5 mm and 3/32" or 2.5 mm. The number of contacts on the jack and socket change according to application: 2 for mono, 3 for stereo, and 4 for additional data, such as microphone input.

The design ensures continuous contact, even when the jack is rotated 360 degrees. This simple design feature has ensured it's continued use for more than a century, and even with the advent of wireless technology.

So the next time you plug in your headphones, spare a thought for the humble TRS Jack and Socket.

Note: This article was written a couple of years before manufacturers started ditching the headphone jack on their devices. And I’m still bitter about that.