Abstract: Process for producing a joint between two metal wires for use in the construction of thermocouples or neutron flux probes or sensors having a small external diameter is disclosed.A stack is formed from two wires placed end to end and having an interface. This stack is placed in a tube which is sealed at both ends. The tube is made from a material, whose thermal expansion coefficient is well below that of the two wires. Spacers or shims are placed between the free ends of both wires. The assembly is exposed to a temperature such that the two contacting ends of the wires are welded under the action of the difference in the elongations of the tube and the wires.