Abstract: Two tubes of thermo-plastic material are joined by a sleeve in an inner portion of which there is an electrical resistance coil, the ends of which are connected to terminals projecting from the outer surface of the sleeve. An electrical circuit supplies electric current to the coil to heat an inner weld zone to welding temperature, while leaving enveloping outer portions at lower temperature. In the sleeve there is provided a radial hole or recess into which the heated material expands upon reaching welding temperature. The material expanding into the hole or recess when welding temperature is reached is sensed by a pressure responsive micro-switch or electro-optically by an optical transmitter and receiver and through a control circuit, shuts off the power to the resistance coil. The parts being thereby fused together, are allowed to cool to complete the weld.