pressure increases the central part of the diaphragm 17 moves towards the left and acts on the movable member 14 through a drive piston 19 so as to tend to move it away from the travel limit position in which it closes the contact 15a 16a this movement is opposed by a helical spring 20 interposed between a screw 21 the function of which will be explained below and an annular flange 22 of a bush 23 a pin 24 of insulating material is force fitted in the bush 23 and has one end projecting axially from the bush and acting directly on the movable member 14 each movement of the movable member 14 towards its second end position that is towards the left with reference to fig2 is thus opposed by the helical spring 20 when the pressure within the tire falls below a predetermined threshold the spring 20 urges the movable member 14 to its first travel limit position in which the electrical contact 15a 16a is closed the electrical signal thus generated is transmitted to the radio transmitter and from this to the receiver located within the passenger compartment of the motor vehicle a further helical spring 25 is mounted coaxially outside the spring 20 and is interposed axially between an abutment ring 26 and a ring nut 27 the ring nut 27 is screwed into a threaded end portion 28 of the inner wall of the tubular member 11 while the screw 21 is screwed into a central threaded hole in the ring nut 27 the threads of the screw.