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In my answer to the question about the LANL MANIAC I posted a picture of it in its natural habitat. In the picture there's a mystery device electrical-taped to one of the columns by the system console. Can anyone identify what it is and say why it would be found in a machine room?

MANIC I with object circled

Source: LANL via Flickr

Here's a perspective-corrected close-up:

Close up of device

The perspective correction makes it look deeper than it really is.

Estimated size: 30 cm (12") wide × 30 cm (12") high × ~4 cm (1½") deep


My best guess is that it's a hygrometer, perhaps to help keep the Williams tubes happy?

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  • Object also visible in the same scene shot from a more advantageous angle in an image at the Computer History Museum. Unfortunately the museum does not seem to have a high-resolution version of this image available on the website.
    – njuffa
    Dec 3, 2021 at 10:27
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    I'd guess hygrometer or similar, too. It was fairly common in later air-conditioned machine rooms for there to be a pen-recorder device monitoring temperature, humidity, &co. Dec 3, 2021 at 12:48
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    @njuffa Here is a higher resolution version of the CHM image: icdn.digitaltrends.com/image/digitaltrends/…
    – jpa
    Dec 3, 2021 at 18:04
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    The similarity of the hole pattern to the paper tape reel visible at the table edge makes me wonder if it could be a backup tape. Perhaps some kind of system diagnostic program that was important to be always at hand but not easily lost :)
    – jpa
    Dec 3, 2021 at 18:28
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    A lot of these comments seem to ignore the fact that the mystery object is strapped to a column with black electrical tape. A "tape winder?" A "tape cartridge?" Wouldn't those things be on a desk or on a shelf or in a drawer? "Strapped to a column, up high" sounds like something that the operators would want to look at from time to time. It doesn't look like any kind of clock, but "hygrometer" is starting to sound pretty believable. Dec 4, 2021 at 19:32

1 Answer 1

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Note: I'm answering my own question here. If someone can verify my findings in an answer I'll accept it.

Williams tubes (a.k.a. electrostatic cathode-ray storage tubes) were used for memory in the MANIAC Ia and the IAS machine on which it was basedb. These were small CRTs that stored bits as static electric charge on the front of the tube1. The addition of a thin metal sheet to the front of the tube allowed the stored values to be read2. Relying, as they did, on static electricity meant that they were very sensitive to environmental conditionsc, primarily humidity3. Too much humidity will prevent static charge from building up and too little humidity can cause excess charge build-up (or even arcing in high-voltage CRT circuitry). This was noted by a Princeton alumnus who remarkedd4:

The 40 CRTs (Williams Tube memory) had some strange characteristics. … [T]hey were very humidity-sensitive. On dry nights they would acquire “spurious bits” — 1s that should have been 0s.

A hygrometer in close proximity and similar conditions to the Williams tubes (i.e. two meters up between the console and the tubes) would thus be invaluable to the machine operators.


1 Omitting the sense plate from a Williams tube and connecting its input to another tube's input would allow the bits stored in that tube to be viewed by the operator. Two such display tubes can be seen in the photo at the left and right of the row of tubes. Here's a close-up (display on the left, storage on the right):

Close-up of Williams tubes

The actual tube is visible as the black and green square at the bottom of the device.

2 Put simply, if a write is done to a location on the tube then the voltage seen on the plate will differ depending on whether a 0 or 1 had previously been stored there. See the linked Wikipedia article for details of how this works.

3 Apparently vaguely close lightning strikes could also cause bit flipsc.

4 Earlier in their remark they misidentify the IAS machine as the very closely-related MANIAC. IMO this in no way detracts from their statement.


a Harlow, Francis H. and Metropolis, N., "Computing & Computers Weapons Simulation Leads to the Computer Era", LA-UR-83-5073, Los Alamos Science, LANL, Winter/Spring 1983

b "Electronic Computer Project", https://www.ias.edu/electronic-computer-project, retrieved 2021-12-03

c “The ­sensitivity of the memory, that was a big problem … If there was a storm with lightning, you would feel it in loss of bits. We spent many nights on the floor trying to tune it up.”
— Estrin, Gerald as quoted in "Daybreak of the Digital Age", https://paw.princeton.edu/article/daybreak-digital-age, retrieved 2021-12-03

d Clarkson, Bill, Comment in response to "Daybreak of the Digital Age", https://paw.princeton.edu/node/53751, retrieved 2021-12-03

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    Never seen a hygrometer like that, despite having them seen an used in may computing rooms of the 70s/80s. Back then they were usually pen recorders, as one want's to not only see random momentary values but record them. They were based on a clockwork driven cylinder and a operated of hair (Keratin reacts reliable to water) pulled by a spring for tension. This construction allowed the force needed for writing to be 90° to the force excreted by the hair/spring system.
    – Raffzahn
    Dec 3, 2021 at 13:58
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    Yea, it looks a little weird. Judging by the electrical tape it could be that one of the operators just went to the hardware store, picked up whatever hygrometer they had in stock, and stuck it up on the column. Dec 3, 2021 at 14:09
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    Whatever it is, it looks a lot like a bathroom scale with part of the lid ripped off, exposing the dial underneath. Dec 3, 2021 at 18:13
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    I think it is a chart recorder, but not a strip chart recorder. There were (probably still are) chart recorders that wrote on a rotating disk; the stylus moved like a phonograph arm. They could be electrically driven or clockwork. The chart record in the picture is not in use, because it has no paper on it. Instead you can see the bare disc that would hold the paper when it is in use. Dec 3, 2021 at 21:07
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    @WayneConrad, OK, maaaaaaaybe a chart recorder, but I question whether it's thick enough to house the clockwork that would rotate the disk. Also, the fuzzy, red blobs that I see through the upper window look like they could be stylized, big-font, display numbers. I'm subscribing to the (non-recording) hygrometer theory. Dec 4, 2021 at 19:36

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