| http://www.w3.org/ns/prov#value | - The space within the casing 8 is evacuated to a suitable degree, for example, to a pressure less than 10- atmosphere and a gas, for example hydrogen is admitted to said space in regulated manner so as to maintain the desired degree of vacuum and at the same time supply a sufiicient number of molecules for production of the ions in the de-' sired amount.
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