The DLS15 is a bi-metallic strip delay switch. The switch contacts operated and HT was applied to the protected circuit after the pre-set delay caused by the time the heater took to heat the bi-metallic strip to the point where the differential expansion of the metals caused it to bend sufficiently to contact the fixed post.
See DLS16 for the same switch but with a 6.3 Volt heater.
The wide glass tube envelope is 30 mm in diameter and, excluding the B4 base pins, is 55 mm tall.
References: Data-sheet & 4049. Type DLS15 was first introduced in 1949.
Absolute Maximum Operating Conditions¶
Updated January 31, 2013. shape:wide.glass.tube construction:pinch type:thermal.delay.switch age:1940.1950 base:b4 heater:4.0v pins:4 pin:1.fc pin:2.mc pin:3.f pin:4.f