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A character in one of my stories wants to steal a valuable artifact from a museum. The artifact is on top of a sensitive pressure plate, which is configured to trigger an alarm if the weight changes for more than a few seconds. This character’s plan is to replace the artifact with another item(s) of identical weight, so as to avoid triggering the alarm.

The problem is, the character does not know the exact weight of the artifact—no one does. How can he remove the artifact without triggering the alarm?

Assume that that this world is identical to ours, pre-Industrial Revolution.

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closed as off-topic by JBH, Agrajag, elemtilas, bilbo_pingouin, Mołot Feb 19 at 8:46

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  • $\begingroup$ Make that an answer @TylerS.Loeper. $\endgroup$ – Cyn Feb 18 at 20:43
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    $\begingroup$ Does pre-industrial mean pre-electrical? Is the alarm system and trigger strictly mechanical? Is this mechanical system accessible? Or is it entirely 'black box technology', hand-waved away? $\endgroup$ – Justin Thyme the Second Feb 18 at 22:41
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    $\begingroup$ For instance, if it is mechanical, and it is counterbalanced, can we assume that as the weight is lifted off, the counterbalance causes the plate to rise? $\endgroup$ – Justin Thyme the Second Feb 18 at 23:07
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    $\begingroup$ Just beware of a giant-rolling-ball trying to crush the character if the replacement's weight is off a bit. $\endgroup$ – user535733 Feb 19 at 0:12
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    $\begingroup$ I apologize for being a party pooper, but what rule of your world are you asking about? Questions about the actions of individual characters are off-topic as too story-based and you've told us nothing about the nature of the alarm other than it's pressure based. There are the proverbial thousand ways of making such a device, and the solution to your problem is based on that story-based decision. VTC OT:TSB. (Although OT:NAW would be just as valid.) $\endgroup$ – JBH Feb 19 at 0:32
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I don't even know how I got to this webpage in the middle of the workday. The aversion to work is great today!

Precision Sand Swap

I'll expand on the sand idea by Cyn to explain how you would synchronize the rate of sand pouring with object lifting and put the correct amount of sand on the plate with the use of a fulcrum or pulley system.

Supplies

  1. Fulcrum or possibly a pulley system
  2. Counterweight greater than weight of object
  3. Sand collecting bucket
  4. Bag of sand exactly equal to the weight of the counterweight minus the weight of the bucket.
  5. Possibly some sand collecting hardware like a funnel or tube and some sort of apparatus like a y-shaped piece of metal to keep the bag of sand separate from the object.

Procedure

  1. Set up the base of the fulcrum next to the object and platform with one end (A) over the object and the other end (B) away from the object. Attach the weight to end B. Attach the bag of sand to the end over the object (A). The counterweight is slightly heavier so you'll have to hold it level.

  2. Now simultaneously attach end A to the object while placing the bucket on the platform. Here is where you may need a method to keep the sand from disturbing the object. The bucket will add weight to the platform, but the fulcrum, with the bag of sand lighter than the counterweight by exactly the weight of the bucket, will lift the object to exactly counteract the weight. This is the riskiest part of the operation as you will be directly disturbing both the plate and the object. There is no way to avoid a certain amount of weight fluctuation here. Be a ninja and hold your breath. The rest is easy!

  3. Assuming that the bag of sand is over the bucket (if not use a funnel or tube), cut a hole in the bag. The sand will pour into the bucket, increasing the weight on the platform. At the same time, the total weight on end A of the fulcrum is decreasing, so the counterweight begins to pull the object off of the platform at the same rate.

  4. As soon as the object is lifted off of the platform, immediately plug or move the bag to stop it from spilling more sand into the bucket.

This solution depends on the pressure plate being bigger than the object being stolen.

It does not solve the problem of weight distribution across the plate if there are multiple sensors underneath, perhaps one at each corner. In this case even though the overall weight stays the same, the distribution across sensors could change and trigger the alarm.

Note also that the counterweight and sand (plus bucket) being equal assumes that the arms of the fulcrum are equal length. You can adjust weights and arm lengths of the initial setup as long as the fulcrum stays balanced.

And... back to work.

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    $\begingroup$ This is a really, really nifty solution. It does not require you to know the weight of the object, it only requires you to have enough sand GREATER than the weight of the object. To increase accuracy, the sower the sand empties into the bucket the more control you have. $\endgroup$ – Justin Thyme the Second Feb 18 at 23:59
  • $\begingroup$ I would love to see an adaptation of this in a heist movie. Also, the sand should be as fine as possible. Instead of using a bucket, if the plate is large enough, the bag should be attached to a small hose with a controllable nozzle so that the sand can be controllably deposited slowly, and by moving the nozzle the weight can be spread over the surface of the plate. I can imagine someone designing a more high-tech and precision version of this contraption. $\endgroup$ – Kai Feb 19 at 0:49
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Fix the plate in place.

What the best way to do this is, will depend on what the pressure plate looks like. However the key fact to remember is that the pressure plate works by being depressed by the weight on top of it. So it is not actually necessary to put something else of exact weight on a pressure plate in order to stop it from firing an alarm. It just has to not lift up when the item is removed.

Options:

  1. Super Glue: If the pressure plate's mechanism is open and available to see then try spraying adhesive to lock the pressure plate in place. This is like spraying super glue into the gears of a clock to prevent the gears from moving.

  2. Make a Mold: Create a mold of the pressure plate that will lock its depression in place. This one is a bit harder because any mold batter you pour on a plate will also have weight, but you could probably lift the item by the same force you are adding, since you know how much batter you are adding. The batter just needs to coil around some external object so that it can rigidly be kept in place.

  3. Two tables and a metal bar: If the item is completely in the open, you can place 2 tables on either side of the plate. Adjust the height of tables until they exactly match the height of the plate. Then use 2 rigid poles (cast iron, wood) that cross over the plate and are attached to each table to lock the plates depression in place. This one adds no actual weight since the tables are supporting the poles' weight. The poles just lock the pressure plate in place by preventing it from moving up.

  4. Other similar ideas. The limitations are only how fancy your thieves' gear can be, and the type of scale you are dealing with.

Those are just some basic ideas. The key concept here is to lock the pressure plate in place, rather than trying to match an unknowable weight.

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    $\begingroup$ I am reasonably confident that none of these would work on even our low-fidelity pressure sensor because the motions involved are simply too small. More to the point, trying the two tables trick will surely set it off. $\endgroup$ – Joshua Feb 18 at 23:20
  • $\begingroup$ Well it is a pre-industrial pressure plate. But if we are using modern technology, we might want to use a laser pointer to get the exact height, and some kind of extremely precise motor to adjust a barrier into place. Presumably leeway on the sensor is also more than a few grams since the item is described as an "artifact", so probably pretty heavy (several kg). If the range was too small the alarm would be going off all the time by accident: air pressure changes, wind, dust, etc. But again we are not told the exact scenario, so it is just a general answer, with basic examples. $\endgroup$ – Tyler S. Loeper Feb 19 at 14:23
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Sand

This is hardly original, I'm sure I've seen it in various movies. Fill a small bag with sand, enough to be more than the weight of the object (have a friend holding another bag just in case it turns out to be lead). Pierce the bag and let the sand slowly pour out as you gently lift the artifact (with straps or help from another human if it's large). If the area with the pressure plate isn't very large, make the first weight transfer a properly shaped container to hold the sand and fit perfectly on the pressure plate next to the artifact.

Because you're writing the story, this difficult task will turn out perfectly right. Or right until the very end when suddenly the thief goofs, the alarm goes off, and the thief grabs the item and runs (which is what s/he should have done in the first place).

Any pre-industrial alarm system will have a bit of flexibility. Either the weight doesn't have to be exact, or the time it takes to make the alarm sound, or the time for the guards to arrive, is just barely enough for the thief to get away (or almost get away, depending on your story).

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    $\begingroup$ Indiana Jones :) $\endgroup$ – John Feb 19 at 4:29
  • $\begingroup$ @John Ding ding ding we have a winner! :-D $\endgroup$ – Cyn Feb 19 at 5:06
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Go buy a luggage hand scale (or custom make one for greater sensitivity). Attach it to the item and as you lift it up, add extra weights to the pressure plate to balance it out. You can use anything for a weight as long as you can add it in increments. So sand and iron weights would allow you to basically get the exact weight you want.

If you have an even better setup, you could attach the scale to a solid base that doesn't move and lift the item with a motor and have it auto dispense the weight it is lifting.

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    $\begingroup$ I don't think luggage hand scales existed before the Industrial Revolution... $\endgroup$ – k2col Feb 19 at 0:53
  • $\begingroup$ @k2col You could make a rudimentary one with just a spring and hook/string and use the displacement of the spring to determine the weight. $\endgroup$ – Shadowzee Feb 19 at 1:54
  • $\begingroup$ There are historical references for spring scales existed (according to Wikipedia en.wikipedia.org/wiki/Weighing_scale#History) at least back to 1770, which is pretty close to the beginning of the industrial revolution (~1760), It wouldn't be too much of a stretch to move the invention date back a few decades. $\endgroup$ – Penguino Feb 19 at 3:38
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Sync's Precision Sand Swap will work if you have enough space on the pressure plate to dump the sand, and if you can ensure the two sand flows are at identical mass per second. But a much simpler method is available if the "weight changes for more than a few seconds" comment in the question is valid. Either bring a spring scale or a two-pan balance in which one pan is swapped with an attachment that can be tethered to the top of the object.

With the spring balance, just lift the object fairly quickly to determine its weight, then gently drop it down again. Should be possible to do this in less than 'a few seconds'. Then Make up an identical weight (sack of sand, brass weights etc.) and quickly lift the object and place the weights. Once again the "few seconds" grace should ensure the alarm doesn't ring.

With the two-pan balance. Connect the first side of the balance to your mystery object. Place a small weight in the second side of the balance while holding it up by hand. Let go of the balance and see if the object rises off the plate. Then, de-weight the second balance. Repeat, increasing or decreasing weight two till it exactly balances. Then, once again, the "few seconds" will give you time to swap the two weights.

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A beam balance, some balloons, and a weight set

Since the alarm takes a few second to go off, we can use a risk free solution!

First, attach a balloon to a beam balance, so that the whole configuration is neutrally buoyant (you will probably want a tank of helium or hydrogen to adjust the balloon's size). Now, remove the artifact, put the beam balance on the pressure plate, and then place the artifact on one of the scales.

Now, you can place test masses on the other scale to determine the mass of the artifact. Note however that you must also attach more balloons to the balance, or the more weight will be exerted on the pressure plate. Therefore, before adding a test mass to the scale, blow up a balloon that neutralizes the buoyancy of the test mass. Note that since the pressure plate is slow to react, you do not need to put the test mass on the scale and attach the balloon to the balance at exactly the same time. Just remember that when removing a test mass, you must detach its balloon. I recommend using different colors for all the balloons to help you keep track of which balloon goes with which weight. This also adds variety.

Once you determine the mass of the artifact using the test masses, you can just dump the test masses onto the pressure plate and remove the balance (along with the balloons and the artifact). Additionally, you can use the test mass neutralizing balloons to neutralize the weight of the artifact. This is not technically necessary, but that way instead of carrying the artifact and scale home, you can just pull them home, which will be much easier on your arms.

If you do not want to abandon the test weights, you can bring a second scale so you can find another object with the same mass of the artifact.

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You need balloons, a tank of helium, and a bucket of sand.

Affix the balloon with neutral buoyancy balloon to the McGuffin with the hose to the helium still attached.

Gradually add equal amounts of buoyancy to the balloon and sand to the pressure plate -- you will need accurate metering of each of course.

As you slowly add to the system, you will reach a point where the McGuffin begins to slowly rise, and you stop adding sand when this happens.

You don't even need to allow for a few seconds of imbalance, just a accurate metering system for the helium and sand. Besides, it is really cool in the movie version.

You don't ever have to know the weight of the McGuffin and you can experiment and refine the system in your own lab till you get it right.

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  • $\begingroup$ It should be possible to do the same with a pulley system, which seems more likely to be available in the setting than helium tank and balloons. $\endgroup$ – R.. Feb 19 at 2:52

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