Interchangeable screws

To prevent interchange of fittings between cylinders of combustible and non-combustible gases, the valve outlets are screwed left-hand thread and right-hand, respectively (Table... 

To prevent interchange of fittings between cylinders of combustible and non-combustible gases, the valve outlets are screwed left-hand and right-hand thread, respectively (Table 9.2). Primary identification is by means of labelling with the name and chemical formula on the shoulder of the cylinder. Secondary identification is by use of ground colours on the cylinder body. Unless specified in Table 9.2, gas and gas mixtures shall be identified by a colour classification indicating gas properties in accordance with the risk diamond on the cylinder label e.g. 

The need for standardisation arose early in the evolution of the modem engineering industry Whitworth introduced the first standard screw thread to give a measure of interchangeability between different manufacturers in 1841. Modem engineering standards cover a much wider function than the interchange of parts. In engineering practice they cover ... 

The Buchner and Hirsch funnels, and the filter flasks and tubes, are available with interchangeable ground glass joints (for example, Fig. 2.43(/)). The Buchner funnel is also available with a screw-thread side-arm for easy attachment via a plastic hose connector to the vacuum line (Bibby Science Products). 

For the crystallographer, one of the most useful ways to describe unit-cell symmetry is by equivalent positions, positions in the unit cell that are superimposed on each other by the symmetry operations. In a P2, cell with an atom located at (x,y,z), an identical atom can be found at (-x, -y, /2 + z), because the operation of a 2j screw axis interchanges these positions. So a P2t cell has the equivalent positions (x, y, z) and (-x, -y, V2 + z). (The V2 means one-half of a unit translation along c, or a distance c/2 along the z-axis.)... 

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