Capillary tube
A soft copper tube with a small inside diameter. The capillary tube used with vapor-pressure thermometer connects the temperature sensing bulb to the Bourdon tube. The capillary tube is protected from physical damage by enclosing it in a braided metal wire jacket.
Also defined in 1 more source
A glass or metal tube with a tiny inside diameter. Capillary action causes the fluid to move within the tube.
Aviation Maintenance Technician Handbook — Powerplant, p. G-5