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Mercury Pendulum

Thurlowe Riley

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Jan 28, 2021
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I Have a Clients Gilbert movement with a Mercury Pendulum. The tops of the Mercury viles are open and only covered by the brass cover. Should I seal the viles and how would I do it? Will the Mercury react with any chemicals like a rubber stopper?
And will sealing the tubes cause pressure from the fumes?
While I am working on the clock I have covered the viles with tape. If I lay the Pendulum down the Mercury will spill out. The customer has always moved the movement with the pendulum sitting strait up in the car but there has to be a way to seal the tubes.

IMG_3010.JPG IMG_3011.JPG
 

LaBounty

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Hi Thurlowe Riley-

I wouldn't try to seal the vials. The mercury in the vials expands and contracts with temperature changes and the air above the mercury needs to be pushed out or sucked in. In other words, the vials must breathe. If the vials are sealed without evacuating the air, the seals (or the vials) will break due to the pressure. The mercury is safe if kept in the vials and not heated to the boiling point. Just be careful not to tip them over :).

Hope that helps!
 

novicetimekeeper

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Barometer tubes are sealed with cloth caps, they allow air through but not mercury. I still would not lie them down but it does prevent accidental spillage.
 

shutterbug

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I think mine have thin cork seals on top. Maybe 1/8" thick.
 

novicetimekeeper

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I think mine have thin cork seals on top. Maybe 1/8" thick.
Unless we get really extreme climate change I doubt the pressure inside will be sufficiant to blow off a cork or damage a tube. Barometers have breathable seals because they can't measure air pressure without them. Many clockpendulum vials are permanently sealed glass capsules. Obviouslt have the glass hot enough to do that with mercury inside is a bit hazardous for the glass worker.
 

Uhralt

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The glass wall of the sealed vials is rather thick in relation to the amount of mercury in them and there is also a rather large volume of air above the mercury that can be compressed by the expanding mercury without a lot of pressure building up inside the vial.
In the large pendulums with a lot of mercury in the open glass containers the wall thickness is much less compared to the amount of mercury in them and there isn't a lot of airspace above the mercury. If this container would be sealed at the top, the container would have to take a lot of pressure because of the little airspace and might break.

Uhralt
 

novicetimekeeper

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The glass wall of the sealed vials is rather thick in relation to the amount of mercury in them and there is also a rather large volume of air above the mercury that can be compressed by the expanding mercury without a lot of pressure building up inside the vial.
In the large pendulums with a lot of mercury in the open glass containers the wall thickness is much less compared to the amount of mercury in them and there isn't a lot of airspace above the mercury. If this container would be sealed at the top, the container would have to take a lot of pressure because of the little airspace and might break.

Uhralt

These are not thin walled tubes.

Lets say the temp rises 20 deg C above the temp at which the vials are sealed.
The increase in temperature is 20[SUP]o[/SUP]C and look at the effect that has on 0.1cc of mercury.

The change in the volume of the mercury is
ΔV = βVΔT = (1.8*10[SUP]-4[/SUP] ([SUP]o[/SUP]C)[SUP]-1[/SUP])(0.100 cm[SUP]3[/SUP])(20 [SUP]o[/SUP]C) = 3.6*10[SUP]-4[/SUP] cm[SUP]3[/SUP] = 0.36 mm[SUP]3[/SUP]


So the volume of the mercury has increased by 0.36 cubic millimetres,

I'm no mathematician but I think that says the volume has increased by 0.36%

Now I think that 50 cubic mm of air will expand by about 6% going from 20 deg C to 40 deg C, and the mercury has not really done anything significant compared to that.

My maths may be way off but I think my 50mm3 of air that was originally at 15psi at 20C is now still just under 16psi 40degC

If I'm right I don't see that thickness of glass wall having a problem with that.

Mind you I've had to to use the coefficient of expansion for mercury, and both Charles law and Boyles law for the behaviour of the gas plus a number of assumptions but I chose a very small air volume because it was suggested that made it worse.

Anybody like to check my maths? I was surprised it was so small

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Uhralt

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These tubes are indeed quite substantial. I have seen much thinner ones in American clocks. Your math is correct. The point I wanted to make is the importance of a certain volume of air above the mercury so the the compressible air will reduce its volume when the non-compressible mercury expands. When you take the theoretical case of mercury in a closed vial with no airspace at all, the expanding mercury, which is much less compressible than air, would put all the pressure directly on the glass container. In practice, a clock vial will always have some space for the mercury to expand, otherwise the compensating effect couldn't take place. A plug on top of the container would probably be enough to withstand the slight increase in pressure as long as there is room for the mercury to expand.

Uhralt
 

novicetimekeeper

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I don't think you would need a large volume of air. Air readily compresses.
That's why I chose such a small air gap for my calculated example. As you can see the expansion of the mercury is minimal compared to the air, so it is the air that is responsible for the increase in pressure, but as the mercury is incompressible there needs to be some air in there (or a vacuum as in a barometer)
 

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