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A while ago I took apart my grandpa's really old radio receiver with vacuum tubes. I recognize everything except for this one strange component. It seems to have some sort of heat sink on the outside and it has a cylinder-like shape on the inside. On one of the sides, it says 6846446 and + and Zenith and 212-23. The + seems to tell me that it is some sort of polarized component but I have no idea what it could be. Here is a picture:enter image description here

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    \$\begingroup\$ I can see a red wire and something that looks like a diode (small black cylinder) but that dark square thing above them doesn't come thru in the photograph very well. Have you tried googling for old circuit diagrams of the radio receiver. There may be one on the net. \$\endgroup\$ – Andy aka Mar 28 '13 at 18:23
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    \$\begingroup\$ Looks like shag carpet from the 60's! (Just kidding.) The pic is too dark and blurry to see anything that would be useful. Can you take a better pic? While you are at it, include a ruler or other common object so we can get a sense of scale. \$\endgroup\$ – user3624 Mar 28 '13 at 18:24
  • \$\begingroup\$ I think the photo is plenty adequate to recognize the component. But no, it'll not get onto the cover of National Geographic. \$\endgroup\$ – DarenW Mar 28 '13 at 18:33
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    \$\begingroup\$ How come you couldn't see for yourself this a small dark blue thing in the middle of a large picture!? The photo is central to your questions, so you really could have give it more care. For starters, don't put your head in the way of the only useful light source. Even then, there is no excuse for not cropping off the irrelevant parts, which is most of the picture. \$\endgroup\$ – Olin Lathrop Mar 28 '13 at 18:40
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    \$\begingroup\$ I enhanced the photo for everyone to see. Now it is good enough for the cover of National Geographic :P \$\endgroup\$ – DarenW Mar 31 '13 at 6:31
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That's a selenium rectifier. Google "selenium rectifier" and there's loads of info, photos. They've been obsolete since the 1970s. Decades ago, they were cheaper than vacuum tube diodes, and semiconductor diodes couldn't handle higher voltages and currents. The parallel plates are indeed for cooling.

http://en.wikipedia.org/wiki/Selenium_rectifier

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    \$\begingroup\$ That's what I think it might be too, but with such a crappy picture it's hard to be sure. \$\endgroup\$ – Olin Lathrop Mar 28 '13 at 18:41
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    \$\begingroup\$ Nice video about a selenium rectifier by that mad aussie bloke (skip to 04:15) \$\endgroup\$ – RedGrittyBrick Mar 28 '13 at 18:53
  • \$\begingroup\$ @Olin: from the textual description I can't image it being anything else. \$\endgroup\$ – Wouter van Ooijen Mar 28 '13 at 19:39
  • \$\begingroup\$ @RedGrittyBrick: Speaking of "mad", did you know that "selenium" is so called because its toxic effects would cause insanity? Selene is the Greek name of the Moon, the Latin name of which ("Luna") is the base of the term "lunatic". \$\endgroup\$ – supercat Mar 28 '13 at 20:52
  • \$\begingroup\$ How many volts do you think it can handle? \$\endgroup\$ – skyler May 14 '13 at 12:30

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