Multistable magnetic cores have the well-known property that a certain amount of flux, stored in the core, remains unchanged for a long time. This amount of flux can be broken down by feeding a number of voltage pulses, with a constant volt-second integral and the right polarity, to a core winding. Each of these readout pulses will lower the flux level in the core until there is no flux left, and from that moment a readout pulse only causes a small reversible change in flux, setting up a very short induction EMF in the core windings. A simple integrating network can be used to judge the output voltage of a winding, and it gives no output if the former results from a reversible flux change. If the pulse generator has a repetition frequence of ten cycles per second, the output pulses of the integrating network, after being shaped by a monostable trigger, will be suitable for dialing purposes in conventional telephone systems. A predetermined amount of flux, corresponding to one of the numbers one to ten, can easily be obtained by discharging a loaded condenser in series with a resistance through a core winding.