the preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings . first , a first embodiment of the present invention will hereinbelow be described in detail . as shown in fig2 a magnetic recording drive of the present embodiment includes , but is not limited to , one or a plurality of magnetic discs 1 , magnetoresistive ( mr ) sensor reproducing composite heads 2 which are provided in correspondence to the number of magnetic discs , an actuator 3 , a voice coil motor 4 , r / w amplifiers 5 , a mode , r / w and head selection control circuit 6 , a write data generation circuit 7 , a reproducing pass selection circuit 8 , an r / w channel 9 , a servo signal reproducing and positioning control circuit 10 , an interface and control circuit 11 and the like . in fig2 the actuator 3 and the voice coil motor 4 are shown in the form of a linear actuator . however , in the case , as well , where a rotary actuator which is widely used in the small disc drives is used for the actuator 3 and the voice coil motor 4 , similarly , this example can be embodied . the mr sensor reproducing composite heads 2 are numbered hdn ( n = 1 , 2 , 3 , . . . , m ). in this connection , the integral number m corresponds to the total mounting number of heads . the mode , r / w and head selection control circuit 6 operates to control the reproducing pass process for the positioning signal and the recorded signal through the reproducing pass selection circuit 8 . by the operations of those circuits , the current flowing time of the sense currents which are caused to flow through the respective reproducing heads each constituted by the magnetoresistive ( mr ) sensor is averaged . the mode , r / w and head selection control circuit 6 includes , but is not limited to , a decoder and reproducing time control circuit 15 and a reproducing time difference memory circuit 16 which operate to analyze an r - w control signal 14 sent from the interface control circuit 11 . the decoder and reproducing time control circuit 15 operates to perform , on the basis of the information in the reproducing time difference memory circuit 16 , the control of the positioning signal reproducing in both the seek operation and the following operation so as for the reproducing time of the mr reproducing heads to be uniform . therefore , the decoder and reproducing time control circuit 15 generates a signal 12 which is used to perform the control of the operation of the r / w amplifiers 5 and the suitable control of the heads , and a signal 13 which is used to control the reproducing pass selection circuit 8 . as an example of the reproducing time difference memory circuit 16 , there is shown in fig3 the specific case where that circuit 16 is constituted by a plurality of up counters 17 which are provided in correspondence to the heads . in this connection , each up counter 17 has the nonvolatile characteristics in order to store the data relating to the accumulated time . now , the description will hereinbelow be given with respect to a method of selecting the above - mentioned servo heads with reference to a flow chart of fig4 . after turning the power source of the drive on , in general , the directory information is read out by the head hd0 , and then is transferred to a processor . the data relating to the reproducing time is stored in the reproducing time difference memory circuit 16 by using both an id signal reproducing count signal 23 and a sector recorded signal reproducing count signal 24 . a position signal reproducing count signal 22 , the id signal reproducing count signal 23 and the sector recorded signal reproducing count signal 24 are , in correspondence to the respective lengths , input to the corresponding bits in each counter . next , until the recording or reproducing in the first head is started , the following operation is performed for a fixed period of time while increasing successively the head number . for that period of time , by the position signal reproducing count signal 22 which corresponds to the following time every head and is generated every sector , the data relating to the reproducing time is stored in the up counters 17 corresponding to the respective heads . the first head hdi ( 0 ≦ i ≦ m ) is selected , and both the seek operation and the r / w operation are performed . for that period of time , in correspondence to the seek , the waiting of rotation , and the positioning time , id reproducing time and the disc storage information reproducing time in the r / w operation in the head hdi , the content of the counter of the head hdi is counted up . at the time point when the reproducing operation has been completed , the ( i + 1 )- th head hdi + 1 is selected . then , after the following operation has been performed for a fixed period of time , as long as the processing does not enter into the r / w operation , the count - up is performed one by one in such a way that the counters are switched successively in the order of hdi + 2 , hdi + 3 , hdi + 4 , . . . , hdm . subsequently , the count - down is performed one by one in such a way that the counters are switched in the order of hd ( m - 1 ), hd ( m - 2 ), . . . , hd1 and hd0 in turn . in addition , as long as the processing does not enter into the r / w operation , the processing is returned to the count - up process , and then the above - mentioned processes are repeatedly executed . in this connection , in order to make the reproducing time in the individual heads fixed , the switching of the heads needs to be performed so as to perform the following operation except for the head or heads having the larger reproducing time . for this purpose , by utilizing the information of the counters 17 in which the data relating to the time elapsed to perform the r - w operation for the heads is stored , the most significant bit of the reproducing time the data of which is stored in the counters 17 is detected by a circuit 19 for detecting the longest reproducing time which is provided in the reproducing time difference detection circuit 18 , and the data relating to the head showing that the most significant bit is &# 34 ; 1 &# 34 ;, i . e ., the reproducing having the longest reproducing time has been performed is shown to a longest reproducing time head storage register 20 . then , except for the head which has been shown by a longest time head register output signal 21 sent from that register 20 , the count - down operation and the counting - up operation are performed with respect to other heads hd . the fixed following time can be selected from the time required for the disc to make one rotation to 10 sec or so . however , in terms of the fine reproducing time control , as for the fixed following time , the time interval which is as short as much as possible is desirable . the above - mentioned embodiment has been described by taking the fixed length recording method , which is widely used in the small disc drives for personal use , as an example . however , it is , of course , to be understood that the above - mentioned embodiment may also be applied similarly to the architecture in which the positioning signals are embedded at regular intervals in the variable length recording architecture which is widely used in the disc drives for the main frame . a second embodiment is constructed in such a way that in the magnetic recording drive having the same structure , shown in fig2 as that of the first embodiment , in order to reduce the influence of the reproducing time in the following operation , the magnitude of the sense currents which are caused to flow through the respective mr heads in the following operation is greatly reduced . an example of the relationship between the sense current density j and the head useful life tf is shown in fig5 . the graph shown in fig5 is obtained from the change in the current flow dependency of the head resistance , plotted for the sense current density as a parameter , shown in fig6 . as apparent from fig5 the following relationship is established . from expression ( 1 ), in order to increase the head useful life tf two - fold and to reduce the influence of the following time , the sense current density j may be multiplied by 0 . 4 , i . e ., approximately halved . in addition , in order to further reduce the influence of the following time , if it is an object that the head useful life tf is increased three - fold , it is enough that the sense current density j may be approximately quartered . as a result , although the s / n ratio in the positioning is also decreased , the decreasing rate thereof is in the range of 1 / 4 or so to 1 / 2 or more . in addition , the high positioning accuracy which is required for the r - w operation is not required for the following operation . therefore , reducing the mr sense current during following operation becomes possible . in addition , in the case of the disc drives suitable for the portable apparatuses , in terms of reduction of the consumed power as well , it is also possible to make the sense currents in the following operation zero . further , the intervals at which the positioning signal is reproduced in the following operation are changed from the intervals every sector in the recording operation to the several - sector intervals , whereby the same effects can be obtained . those methods may be combined with the first embodiment , and as a result , the more advantageous effects can be expected . fig7 shows the structure of the mr head which is employed in the first and second embodiments . this structure is such that an mr film 25 and an electrode 26 are formed through a magnetic domain control film 27 so as to confront each other , and those films are covered with a shielding film 28 . according to the present invention , in the magnetic disc drive in which the magnetoresistive sensor reproducing composite heads are mounted , it is possible to provide the magnetic recording drive in which the control is performed in such a way that the servo heads are switched so as to average the total time elapsed to cause the sense currents to flow through the respective magnetoresistive sensors . as set forth hereinabove , according to the present invention , the time required for the sense current to be caused to flow through one mr reproducing head is greatly shortened . therefore , it is possible to increase the useful life which depends on the degradation of the sensor characteristics due to the electromigration and the like . in addition , if the same useful life is allowed , the amplitude of the sense current can be increased , and the outputs of the mr reproducing heads can be increased without reduction of the reliability . accordingly , it is possible to increase the useful life of the high output magnetic recording drive employing the mr sensors .