How are weights and scales used by the public calibrated? My physics teacher has a set of masses: things from 1 to 10 grams. My chemistry teacher has electronic balances that measures things in grams. In France, there is a block of metal we hope no one accidentally breaks, because its the kilogram.
My question is: how does the mass of the prototype of the kilogram get "transferred" to things like masses and balances used by classrooms?
By what process are my physics-teacher's gram masses manufactured so that they are approximately a thousandth of the mass of the kilogram? and how my chemistry-teacher's balances tuned, so that if the prototype kilogram ( by one of my wacky stunts) were placed on them, they would read "1.000 kg?
 A: When the I nternational prototype kilogram (IPK) was created, copies were made and sent to the most important countries in the world and are kept in a protected environment. 
Periodically they are returned to France, checked and compared and, surprisingly enough, their masses do not match anymore.
Factories that produce these items have access to them or have perfect replicas in their turn, and produce accurate weights and with those weights you can tune your own balances even at home: see here

So its only about 3 or 4 steps then? –  PyRulez

Are you referring to the video? I'll try to clarify, your question referred to manufacturers:

How are weights and scales used by the public calibrated?

As I briefly hinted above, in every developped country there is a National Institute of Metrology and each institution " will hold several "national standard" kilograms, which are periodically calibrated against the IPK and each other."
Manufacturers make/sell a) replicas of this Secondary standards for laboratories and b) accurized scales, so, when 'the public' buys scales, they have been already tuned. Besides that, in town market-places there is usually an office of the municipal police that periodically checks the accuracy of the scales of the sellers, and where 'the public' can verify the accuracy of the weight of purchased goods.
Now, as to your follow up question: most household/ kitchen-scales can be calibrated at home if they have the 'calibrate' function/ button, like the one in the video. It is only one step: after you push the button, you put an accurate (required) weight onto it, and it authomatically calibrates itself. The only problem is to find an accurate weight: If you personally have that problem, go to a pharmacy and check your home-made sample on their precision scales and then use it at home for a couple of months.

So I presume bathroom scales are simply based on tables and spring
  laws then? –  PyRulez

Yes, springs are made according to Hooke's law: they are calibrated so that measured-force translates to mass at earth's gravity. They are becoming obsolete because they cannot be very accurate and they have two sources of error: the measured weight varies with the strength of the local gravitational force (by as much as 0.5% at different locations on Earth), and the elasticity of the measurement spring can vary with temperature. 
Nowadays scales are usually electronic devices: if you want to know more about how scales are actually manufactured and tested, you can find plenty of details on the web: start with the quoted wiki's article and follow the links therein strain gauge, load cell, microbalance, etc.
A: In the US, if you purchase a balance or set of reference weights (masses) or a scale for scientific purposes, you can also purchase with it a certificate of traceability. This is a document that states how your device was compared to a reference, and how that reference was traceably compared to an even better reference, and so on, up to the standard kilogram maintained by NIST, which is the national standards body here. And of course that standard kilogram was originally manufactured and compared to the international prototype kilogram in Paris. 
Similarly, the scales used at your grocery store to weigh goods you buy are probably tested periodically by your local bureau of standards and measures (this is a county-level function in the US) using instruments or standard weights with a traceable derivation back to the national standards body. Uniform measurements for commerce is, after all, the original reason that national standards bodies were set up.
On the other hand, if you buy a consumer grade bathroom or kitchen scale, they probably tested it with a standard weight of some kind, but its unlikely they took extraordinary measures to maintain that reference or to ensure it's accuracy to more than 1% or so.
