Measurements are reported on the color sensitivities of various photoelectric tubes having cathodes made of alkali metals. A composite surface of Rhyphen;Aghyphen;ROhyphen;Ag of any alkali metal R shows a higher sensitivity over a wider range of wavehyphen;lengths than Rhyphen;ROhyphen;Ag and has two peaks, one between 330 and 370 mmgr;, and one at about 500 mmgr; for potassium and 550 mmgr; for rubidium and between 700 and 800 mmgr; for caesium. If a gas filled lamp at 2700deg;K is used as a light source, photoelectric currents from vacuum photohyphen;tubes are obtained sometimes as large as 29mgr;a per lumen for potassium, and 10ndash;15mgr;a per lumen for rubidium, and 40ndash;50mgr;a per lumen for caesium. It is also reported that the potassium cell in combination with a proper filter would constitute a means for an ideal photometer.
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