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Can anyone tell me what is the purpose of the outter dial of this watch? Notice how the scale is not linear and goes to 100. It can only be rotated counter-clockwise only.

This is the only picture I have.

watch

Sorry, it's not really a puzzle but I didn't know which stackexchange site to use. This one seemed to fit.

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  • $\begingroup$ The line is part of the dial, it doesn't grow/shrink it's painted that way. It rotates with the dial. $\endgroup$
    – knarf
    Commented Aug 31, 2014 at 22:27
  • $\begingroup$ This question appears to be off-topic because it is about features of a watch, and is not a puzzle. (It's an interesting question, though, but definitely not on-topic here, sorry!) $\endgroup$
    – user20
    Commented Sep 2, 2014 at 0:35

2 Answers 2

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It is a tachymeter. (Once you know that, the big yellow T makes sense.) This a common enough thing on luxury watches - I didn't actually know this, but this morning I saw a glossy ad for one on the back of a magazine and recognized the scale. If you search tachymeter on Amazon or the like you can see any number of images.

The purpose of it is to determine speed. Going a certain distance in a given time means you are going a particular speed. You would need to learn what units your watch ring is in (km/hr or mph for example) and what the calibration distance is, but then you could say that going that far in about 10 seconds means you're going 30 somethings, or going that far in 20 seconds means you're going 15 somethings.

More expensive watches run the scale to a higher number, presumably because people who buy that kind of watch are in faster cars, jets etc but this only means a longer calibration distance, no change in the capabilities of the watch.

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    $\begingroup$ The scale with respect to the seconds hand is in 1/360Hz, so in order to get the speed you should calculate distance*(1/360Hz)*readout. For instance 100m*(1/360Hz)=1km/h, and 528ft*(1/360Hz)=1mi/h, so if you run 100m or 528ft the scale will be in km/h or mi/h respectively. Note that the timing should start when the hand pass the number 6, rather than the bottom of the T mark. $\endgroup$ Commented Sep 1, 2014 at 20:32
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    $\begingroup$ @eBusiness: I seriously doubt the last sentence in your comment is correct. $\endgroup$ Commented Sep 2, 2014 at 16:44
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    $\begingroup$ @ThePopMachine That is why I wrote it, the design is counter-intuitive. Anyway, it is an inverse number scale, so the numbers have to be inverse proportionate to their distance from the start of the scale. 6 is twice as far from 50 as it is from 100, so 6 must be the starting point. $\endgroup$ Commented Sep 2, 2014 at 17:46
  • $\begingroup$ @ebusiness: 50 is not the same distance from 25 as it is from 100. $\endgroup$ Commented Sep 2, 2014 at 17:52
  • $\begingroup$ @ThePopMachine Of course not, 50 is the same distance from 25 as it is from 6. If you divide in equal distances then you will get fractions with continually increasing denominators: 100/0=infinity (not marked), 100/1=100, 100/2=50, 100/3=33.3, 100/4=25, 100/5=20, 100/4=16.7 etc. $\endgroup$ Commented Sep 2, 2014 at 17:58
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That outer dial resembles the slide-rule dial found on some watches, for example on various Citizen Nighthawk watches. Section H of the pdf manual describes how to use the slide-rule dial to compute arrival times, fuel consumption, etc as well as how to multiply and divide. You can download nighthawk_bj700052e.pdf via manualslib if interested.

The Nighthawk has a fixed nonlinear ring just inside the outer nonlinear ring, which is needed for simple slide-rule function. I don't see a non-linear dial in your photo; in which case the ring might not implement a general slide rule, but instead is specialized for some single function, like ETA.

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  • $\begingroup$ They look similar but it is not what is on the picture. A slide rule goes clockwise from 10 to 90 and then go back to 10 where the 100 would have been. 10 and 60 are highlighted as they serve to convert minutes to hours. $\endgroup$
    – Florian F
    Commented Sep 3, 2014 at 14:23

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