fig1 shows an electromagnetic flowmeter 1 having a measuring tube 2 , the flow through which is perpendicular to the plane of the drawing . the measuring tube 2 is electrically insulated . arranged perpendicular to the direction of flow is a coil arrangement consisting of two coils 3 , 4 , which generate a magnetic field perpendicular to the direction of through - flow when current flows through the coils 3 , 4 . measuring electrodes 5 , 6 and earth electrodes 7 , 8 are provided in the measuring tube 2 . the measuring electrodes 5 , 6 are arranged so that they detect a potential difference or voltage perpendicular to the direction of through - flow and perpendicular to the magnetic field . the voltage between the electrodes 5 , 6 rises in a manner known per se as the speed of the fluid in the measuring tube 2 increases and as the magnetic field increases . the coils 3 , 4 are connected in series and are supplied from a supply source 9 with a voltage vnom , the direction of the coil current being determined by an h - bridge circuit with four switches 10 - 13 , each switch being protected by a free wheeling diode 14 - 17 . when the current is intended to flow anti - clockwise ( as seen in fig1 ) through the coil arrangement 3 , 4 , the switches 10 , 13 are closed . the switches 11 , 12 remain open . when the current direction is to be reversed , the switches 11 , 12 are closed and the switches 10 and 13 are opened . the coil current is regulated by a current regulator 18 , which is to ensure that the current through the coil arrangement is constant . the current i through the coil arrangement 3 , 4 is then passed through a measuring resistor 19 . the measured voltage across the measuring resistor is supplied to a comparator 20 , the other input of which is supplied with a constant voltage that is obtained from a voltage divider comprising three resistors 21 , 22 , 23 with a reference voltage vref at their input 24 . the output of the comparator 20 is connected to a timer 25 . the timer 25 is additionally connected to a control circuit , not specifically shown , which controls the operation of the switches 10 - 13 . for the rest , the measuring electrodes 5 , 6 are connected to a differential amplifier 27 , the output of which is connected to an analogue - to - digital converter 11 . the analogue - to - digital converter 28 is supplied from the same reference voltage source 24 as the voltage divider comprising the resistors 21 - 23 . at its output 29 it produces digital values for the through - flow determined . when the switches 10 - 13 are operated , that is , the direction of the current i through the coil arrangement 3 , 4 is reversed , the time - counter 25 starts to count or measure the time . this time counting is continued until the voltage across the measuring resistor 19 is the same as the voltage across the resistors 22 , 23 . at that time , the comparator 20 sends a signal to the time counter 25 , which stops counting and gives out the determined time at its output 26 . the time at the output is compared with a time determined earlier , for example , during initial operation of the flowmeter . in uninterrupted operation the currently determined time should be the same as the time determined earlier and at any rate should have quite small variations within a tolerance range . if this is not the case , it points to a change in the flowmeter that could signify an error in the measuring result . to compensate for temperature fluctuation , a further resistor ( not shown ) having a negative temperature coefficient can be arranged in series with the coils 3 , 4 . the electrical resistance of the current path from the voltage source 9 to the measuring resistor 19 therefore remains substantially the same , regardless of the temperature , so that no changes are caused thereby . fig2 shows a modified embodiment . the coil arrangement 30 is connected to the h - bridge 32 , now represented merely diagrammatically , which is connected between the voltage source 31 , which supplies a normal voltage vnom , and the measuring resistor 35 , which in turn is connected to earth . the resistors 38 - 40 correspond to the resistors 21 - 23 . the comparator 36 corresponds to the comparator 20 and the timer 46 corresponds to the timer 25 . as a modification to the construction according to fig1 a supplementary voltage supply 44 has now been added , which supplies a supplementary voltage vboost . this supplementary voltage supply 44 is applied via a change - over switch 33 to the h - bridge 32 , specifically , from a time at which the current direction is reversed to a time when the current has reached a predetermined value again . the supplementary voltage vboost is higher than the normal voltage vnom , so that the rise in the current is effected more quickly . the current regulator 34 is , as in the construction according to fig1 also , regulated by a reference voltage across the resistor 40 of the voltage divider 37 . this voltage serves as reference . in addition , the voltage across the resistor 40 is supplied to a further operational amplifier 43 , the output of which is connected to the supplementary voltage supply system 44 . the other input of the operational amplifier 43 is connected to the center tap of a voltage divider comprising two resistors 41 , 42 , which is arranged between the output 45 of the supplementary voltage supply system 44 and earth . the output of the operational amplifier 43 therefore regulates the supplementary voltage supply system 44 , which can also be referred to as the “ boost generator ”. the output of the comparator 36 not only triggers the time - counter 46 , but also the switch 33 , so that in the time after change - over not only is the length of time necessary for the current to reach its predetermined value determined , but also in this time an increased voltage is used . this increased voltage moreover has the advantage that it is relatively accurate . indeed , all voltages are related to the reference voltage vref . the time counter 46 determines , as stated , the length of time that is needed for the rise in the coil current after polarity reversal . this time is a measure of the electrical and magnetic properties of the system as a whole . this length of time is unique to a specific system , a kind of “ fingerprint ”. because the same reference voltage vref is used as the basis for regulating the coil current and the supplementary voltage supply system 44 , a fixed and precise relationship between the supplementary voltage and the coil current can be reached . when this reference voltage is used at the same time to control the analogue - to - digital converter 28 ( fig1 ), then very accurate measurements can be achieved . the measuring accuracy of the flowmeter is improved and at the same time the electrical and magnetic circuits of the flowmeter can be monitored . fig3 shows the waveform of the current i through the measuring resistor 19 in fig1 . on polarity reversal of the current direction , the coils 2 , 3 will first of all try to keep the current at its previous intensity . because of the free - wheeling diodes 14 - 17 , the current through the measuring resistor 19 will briefly change its sign , the coil current dropping and changing direction . fig4 shows the same curve 47 to an enlarged scale , that is , for a shorter period . since the rise in the coil current follows a specific physical natural law , the time period t is a measure that provides information about the rise in the coil current with sufficient reliability and certainty . this time period t should be the same at each change - over , or should vary only by a small differential amount from a set value . of course , it is possible also to take several measurements and to plot the curve 47 point by point , which is advantageously done by a microprocessor , not shown specifically . it is therefore possible to determine not only the parameter t , but in fact to compare a curve shape . in fig5 two curves are shown , of which the curve 47 is produced when the flowmeter is free from defects . in the curve 48 , the rise is too rapid , that is , the time tf is too short . the curve 47 has been included again with its rise time t to make the differences clear . the invention can also be carried out with several stabilised voltages , but in that case they may differ from one another only by relatively small amounts ( a few microvolts ).