for passing from position p3 to position p2 and then to position p1 thus the microprocessor permanently monitors voltage us if the latter increases and passes beyond the top of the measuring range the microprocessor gives the order to switch to the higher measuring range whilst switching the gain from 1 to 10 and switching to the resistor with the immediately lower value if it drops below 10 of the measuring range the microprocessor gives the order to switch to the lower measuring range switching the gain from 10 to 1 and switching to the resistor with the immediately higher value on discontinuing the use of the apparatus and when the operator has placed it in the 34 stop 34 position the microprocessor gives motor m the instruction to rotate to position p5 the apparatus remains energized by interrupter im5a which remains closed for as long as magnet ap has not reached said position p5 when it reaches this position there is a cutting off of the power supply and the short circuiting of the transistor tr1 amplifier ai assembly by closing interrupter im5 this places switch c in the safety position to protect transistor tr1 during the following energizing of the apparatus reference will now be made to fig2 for describing a special embodiment of the switch according to the invention the mechanical construction and choice of components have been made with the aim at obtaining a simple and compact assembly the basic structure is a u shaped body 1 formed from a standard aluminium shaped section motor m is mounted on one of the flanges 2 of said body and its shaft passes through an orifice therein this shaft is integral with a rotary.