Abstract: A capillary tube is shrunk around a wire heating element, fashioned into the desired configuration, and situated between the exterior surface of a cylindrical inner glass tube and the interior surface of a cylindrical outer glass tube, to form an assembly. The space between the adjacent tube surfaces is evacuated as a heat source is moved along the axis of the assembly to heat each longitudinal section of the assembly, in sequence. The element is thermally expanded as the outer tube surface collapses toward the inner tube surface, resulting in the fusion of the adjacent glass surfaces around the expanded element. In this manner, a channel is formed through the fused glass body which has the configuration and dimensions of the expanded element. During cooling, the element returns to its unexpanded condition. This results in a space between the channel walls and the element, permitting the element to expand, when electrically energized, without breaking the glass body.

Abstract: A capillary tube is shrunk around a wire heating element, fashioned into the desired configuration, and situated between the exterior surface of a cylindrical inner glass tube and the interior surface of a cylindrical outer glass tube, to form an assembly. The space between the adjacent tube surfaces is evacuated as a heat source is moved along the axis of the assembly to heat each longitudinal section of the assembly, in sequence. The element is thermally expanded as the outer tube surface collapses toward the inner tube surface, resulting in the fusion of the adjacent glass surfaces around the expanded element. In this manner, a channel is formed through the fused glass body which has the configuration and dimensions of the expanded element. During cooling, the element returns to its unexpanded condition. This results in a space between the channel walls and the element, permitting the element to expand, when electrically energized, without breaking the glass body.