This lovely device is an electronic frequency standard dating from the 1940's or early 1950's, a large tuning fork with a vacuum tube amplifier, and electromagnetic transducers to drive the fork. It outputs a nice sinusoidal voltage; a side affect is the constant, and substantial, singing of the tuning fork in it's Micarta and wood case.
Here's a fairly decent audio recording of it (GR723c.wav). It's quite loud! The audio output is mere side effect of the design; it's main output is electrical; it outputs 7V of 1000Hz sinusoidal across a 10K ohm load (the vertical calibration in the photo below is off 10X.)
This may be the single most exquisitely constructed lab instrument I own. It's exceedingly Old World, entirely hand made from standardized parts. The covers, top and side, made of solid wood shellacked, are held on by machine turned brass screws with a black oxide finish. Could you even buy such things today? The finish on them is fabuluous.
Inside the front cover is a schematic and descriptive text, enclosed in some sort of ageless transparent plastic, it is held to the wooden cover by a dozen tiny wire brads.
The wires to the power supply, fastened to a micarta terminal strip with nickel plated hardware, is stamped and paint-filled. The identifying tags on the wires themselves ("B+", "B-", etc) are made of steel, hand stamped, and crimped on. The amount of care in such a modest location inside such a modest instrument is literally unparseable today. I feel honored just to caretake it.
This webpage is itself a study in aging media; first put up in 1996, my first digital camera had a low-quality megapixel resolution and a cheap plastic lens ("burnt toast and a rotten egg -- I have a tapeworm, it's good enough for it"). Around New Years 2010 David Forbes asked if I could take some better photos of the thing, a friend wanted to know just how could one drive a tuning fork as an oscillator? Well my cheap commodity camera now had needlessly excessive megapixelage; I took 44 photos, two audio recordings on a device that has more transistors in it than God had in 1950, and a couple minutes of HD video to see if that cold catch anything interesting (it did not) thereby invoking in a few minutes more storage than existed around me in a five-block area, back in 1996. Then, of course, I threw most of it away. The remainder can be seen below. Oh yeah, i threw in some quick and dirty measurements with my 'scope just for the hell of it. Not many instruments of today will be operable in a small fraction of this thing's current age; and a quick look shows it's short term stability to be rediculuously stable.