fig1 shows a circuit diagram of a resistor matrix as it is used in a pressure - sensitive resistor mat arranged in a vehicle seat for detecting seat occupancy . the resistor matrix has row conductors and column conductors , which are all connected to one another via pressure - sensitive resistors . in the embodiment illustrated , there are four row conductors lz 1 , lz 2 , lz 3 , lz 4 and five column conductors ls 1 , ls 2 , ls 3 , ls 4 , ls 5 . there are resistors rsz , s being the column index ( s = 1 , 2 , 3 , 4 , 5 ) and z being the row index ( z = 1 , 2 , 3 , 4 ). for example , resistor r 11 connects first column conductor ls 1 to first row conductor lz 1 ; resistor r 32 connects third column conductor ls 3 to second row conductor lz 2 ; resistor r 54 connects fifth column conductor ls 5 to fourth row conductor rz 4 , etc . thus , a matrix of s * z resistors is obtained , which are distributed over the entire seat surface and respond to pressure exerted in the zones of the seat surface assigned to the individual resistors with a change in resistance . in order to detect the change in resistance which is proportional to the pressure for all resistors in the matrix , the measuring circuit described below is used . each column conductor ls 1 , ls 2 , ls 3 , ls 4 , ls 5 is connected to the output of an operational amplifier os 1 , os 2 , os 3 , os 4 , and os 5 . similarly , each row conductor lz 1 , lz 2 , lz 3 , lz 4 is connected to the output of an operational amplifier oz 1 , oz 2 , oz 3 , oz 4 . each of these operational amplifiers os 1 , . . . , os 5 and oz 1 , . . . , oz 4 has a first input feedback connected to the output and a second input which can be connected to a voltage u 1 or a voltage u 2 via a switch . the two voltages u 1 and u 2 are different ; for example , voltage u 1 = 5 v and u 2 = 2 . 5 v . there is a processor pr , which switches operational amplifiers os 1 , . . . , os 5 for the columns and operational amplifiers oz 1 , . . . , oz 4 for the rows consecutively to voltage u 1 or u 2 so that a voltage is applied consecutively to all resistors rsz of the resistor matrix . in the initial condition , operational amplifiers os 1 , . . . , os 5 for column conductors ls 1 , . . . , ls 5 as well as operational amplifiers oz 1 , . . . , oz 4 for row conductors lz 1 , . . . , lz 4 are switched to the same voltage u 2 . then , both terminals of each resistor rsz are at the same potential , and there is no voltage drop across them , and therefore they are conducting no current . then , operational amplifier os 1 for first column conductor ls 1 is switched over to the other voltage ul . all the other operational amplifiers continue to be at voltage u 2 . due to the voltage switch - over . . . , of operational amplifier os 1 , both terminals of resistors r 11 , r 12 , r 13 , and r 14 in the first column are at different potentials , namely u 1 and u 2 , so that there is a voltage drop across them and they also conduct a current . after the currents flowing through these resistors and the voltage drop across them have been measured , as will be described in more detail below , and the resistance values have been calculated therefrom by processor pr , operational amplifier os 1 of first column conductor ls 1 is switched over again to voltage u 2 and operational amplifier os 2 of second column conductor ls 2 is switched over to voltage u 1 . then , the resistance values of resistors r 21 , r 22 , r 23 , and r 24 can be determined . thus , the resistances are measured in all columns . in order to measure the individual resistances in the matrix , the voltage applied to the respective resistor and the current flowing through it is measured . the voltage drop across each resistor is determined by processor pr , to which is connected outputs s 1 , s 2 , s 3 , s 4 , and s 5 of column conductors ls 1 , ls 2 , ls 3 , ls 4 , and ls 5 and outputs z 1 , z 2 , z 3 , and z 4 of row conductors lz 1 , lz 2 , lz 3 , and lz 4 . the currents through the individual resistors are detected by operational amplifiers oz 1 , oz 2 , oz 3 , and oz 4 connected to row conductors lz 1 , lz 2 , lz 3 , and lz 4 . the currents through the resistors could also be detected via operational amplifiers os 1 , os 2 , os 3 , os 4 , and os 5 associated with column conductors ls 1 , ls 2 , ls 3 , ls 4 , and ls 5 . the current that flows through a resistor flows through the row conductor connected to the respective resistor and appears as an output current at the operational amplifier connected to this row conductor . each operational amplifier oz 1 , oz 2 , oz 3 , and oz 4 associated with the row conductors is provided , according to the embodiment illustrated in fig2 of an operational amplifier op , with a current balancing circuit which is capable of detecting output current ia of operational amplifier op , which is exactly equal to the current flowing through the resistor which is about to be measured . output current ia flows into output stage es of operational amplifier op . the design of output stage es will not be described here in detail , since it is a conventional circuit for operational amplifiers that is well known to those skilled in the art . output stage es is normally connected to a positive supply voltage + u and to ground . the current balancing circuit operates by tapping both current i 1 flowing from output stage es to supply voltage source + u and current i 2 flowing from output stage es to ground . if current ia flowing into output stage es is positive , the equation i 2 =−( i + ia ) applies to the current flowing to ground , i being the cross current flowing through the output stage . the equation i 1 = i applies to the current flowing to supply voltage source + u . if current ia flowing at the output of output stage es has a negative sign , the equation i 1 = i + ia applies to current i 1 and the equation i 2 =− i applies to current i 2 . in order to detect output current ia alone , which corresponds to the current flowing through the resistor to be measured , independently of cross current i in output stage es , the two currents i 1 and i 2 are supplied to an adder su . by adding the two currents i 1 and i 2 , cross current i of output stage es is eliminated and only desired output current ia appears at output a of adder su . since the values of the pressure - sensitive resistors fluctuate between 1 kωand 2 mω , current ia flowing through the individual resistors can also change over a very wide measurement range of approximately 5 μa to 5 m a . if this extremely wide range of currents is to be transformed into a narrower current range , which would facilitate the evaluation of the measured currents in processor pr , circuit arrangements mu 1 and mu 2 are provided , using which the two currents i 1 and i 2 supplied to adder su are transformed into a limited defined current range of approximately 250 μa . such conventional circuit arrangements for measured value transformation are known per se and include a plurality of attenuating and amplifying stages . as fig1 shows , outputs a 1 , a 2 , a 3 , and a 4 of operational amplifiers oz 1 , oz 2 , oz 3 , and oz 4 associated with the rows , which provide the currents through the resistors , are connected to processor pr . instead of currents at outputs a 1 , a 2 , a 3 , and a 4 , appropriate measuring voltages can also be supplied to processor pr . the measuring voltage of each operational amplifier can be obtained through a resistor connected to the output of adder su and traversed by output current ia . in order for defects in the resistor mat , for example , broken conductors , to be diagnosed , the resistors in one column and in one row have fixed , pressure - independent resistances . these pressure - independent resistors are preferably in one column and in one row at the edge of the resistor mat . in the embodiment shown , these are resistors r 11 , r 12 , r 13 , r 14 , r 24 , r 34 , r 44 , r 54 . due to the fact that the pressure - independent resistors are located at the edge of the resistor mat , i . e ., outside the actual seat surface , the pressure - sensitive measuring surface is not reduced .