Radioluminescent Gauge (ca. 1950s, 1960s)

As the label on the side indicates, this voltmeter employs a radioluminescent meter face. I assume that it was produced in the 1950s or 1960s because the radiation warning symbol was developed in the late 1940s, and because the use of radium (Ra-226) decreased dramatically after the 1960s. It was manufactured by Wacline, Inc., of Dayton, Ohio. The Ra-226 content is probably 0.5 to 1.0 uCi. In this example, the radioluminescent paint, which has a light tan color, is applied to "A.C. VOLTS," the numbers 0 through 150, their associated hash marks, and the tip of the needle.

Radium-containing radioluminescent gauges have been around at least since World War I when they were employed in aircraft. 

Reference

S.A. Sochocky. Can't You Find the Keyhole? The American Magazine Vol. 91, Jan.-June, 1921.