Three stages in the growth of the valve. Above: One of the original audions of 1912 or thereabouts, hand-made and of fragile construction, gassy, non-uniform and short lived. The valve of 1924 (centre) illustrates the intermediate period. We can see in it the modern valve taking shape.
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https://paperspast.natlib.govt.nz/periodicals/RADREC19310724.2.8.4
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Radio Record, Volume V, Issue 2, 24 July 1931, Page 3
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47Three stages in the growth of the valve. Above: One of the original audions of 1912 or thereabouts, hand-made and of fragile construction, gassy, non-uniform and short lived. The valve of 1924 (centre) illustrates the intermediate period. We can see in it the modern valve taking shape. Radio Record, Volume V, Issue 2, 24 July 1931, Page 3
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