PURPOSE: To improve the high temperature characteristic by encapsulating the amalgam where the mercury is contained in the basic alloy composed of tin, lead and indium in a bulb.

CONSTITUTION: The weight ratio between the basic alloy and the mercury in an amalgam to be encapsulated in a bulb where the mercury is contained in the basic alloy composed of tin, lead and indium is in the range from 0.90:0.10 to 0.995:0.005 while the weight ratio between tin and lead and indium is in the range of 41:34:25 to 34:41:25. The curve A will show the amalgam where 0.75wt% of mercury is contained in an alloy composed of 41wt% of tin, 34wt% of lead and 25wt% of indium, while the curve B will show the amalgam where 0.75wt% of mercury is contained in an alloy composed of 37.5wt% of tin, 37.5wt% of lead and 25wt% of indium while the curve C will show the amalgam where 0.75wt% of mercury is contained in an alloy composed of 34wt% of tin, 41wt% of lead and 25wt% of indium and the curve D will show the vapor pressure curve of the pure mercury. The mercury vapor pressure of three kind of amalgams under the low temperature is near to that of the pure mercury while the mercury vapor pressure under the high temperature is ballanced near to 6×10-3.