Using coins for scale is discouraged as it will require people unfamiliar with them to look up the dimensions or guess, both of which defeat the purpose of the object in the first place. Coins can also reinforce a geographical bias, and some coins' designs are copyrighted.
Ideally, a photograph should include a ruler with the subject (example) or an added scale marking.
SI ("metric") units are the most commonly used worldwide (see meter and centimeter).
DescriptionThree xenon flashtubes and a krypton arc lamp.jpg
English: The first three from the top are xenon flashtubes. The bottom one is a krypton arc lamp, which has a smaller diameter, higher pressure, thinner glass (for better cooling), and a cathode with a sharp tip. The anodes are on the right hand side, and the cathodes are on the left. The second flashtube is designed for water cooling, and portions of the glass can be seen that have been shrunken around the electrodes (also for cooling). This flashtube shows signs of sputter around its cathode, appearing as dark deposits of metal on the glass. A penny is shown for size.
This work has been released into the public domain by its author, Zaereth. This applies worldwide.
In some countries this may not be legally possible; if so: Zaereth grants anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.
({{Information |Description = The first three from the top are xenon flashtubes. The bottom one is a krypton arc lamp, which has a smaller diameter and a cathode with a sharp tip. The anodes are on the right hand side, and the second flashtube shows si)
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