now the present invention will be explained in detail with reference to an embodiment thereof shown in the attached drawings . fig1 is a schematic diagram of a printer embodying the present invention , wherein a servo - motor drive circuit 1 drives a servo - motor 2 through an amplifier 1a . said servo - motor 2 is provided with position detectors 3 , 4 , of which signals are amplified by amplifiers 3a , 4a . thus , the detectors 3 , 4 , composed of slits and optical elements , provide pulses of different timings in response to the rotation of the servo - motor 2 . the servo - motor 2 is mechanically connected , through unrepresented gears , wires etc ., to carriage 5 supporting a thermal head having plural heating elements arranged in a vertical line and performing reciprocating motion along the printing direction in response to the rotation of the servo - motor 2 . the carriage 5 is provided with a connector 7 for detachable mounting of the printing or thermal head 6 and electrical connections therefor . in facing relationship to the heating elements of the thermal head 6 there are provided a recording paper 8 and a platen 9 , with which the thermal head 6 is maintained in pressure contact during the printing operation . a head discrimination circuit 10 is connected to the connector 7 supporting the thermal head , in order to identify the kind of the thermal head mounted . a selecting circuit 11 selects the timing pulses either from the detector 3 or from the detector 4 , in response to the signal from the head discrimination circuit 10 . the output signal from said selecting circuit 11 is supplied to the motor drive circuit 1 controlled by the print instruction signal , thus controlling the rotating speed of the servo - motor . said output signal is also supplied to a head drive circuit 12 to determine the drive time of the thermal head 6 maintained in stand - by state by the print instruction signal and accordingly drive said thermal head . fig2 shows an embodiment for discriminating the kind of the thermal head 6 . the thermal head 6 is composed of a graze area 61 , a heat element area 61a , a printed circuit board 63 connected thereto and a heat sink 62 , wherein said graze area and printed circuit board are screwed to the heat sink to constitute a thermal head unit . the circuit patterns on the board 63 include u - shaped patterns 63a , 63b not connected to the heat elements 61a , in which the pattern 63b has a partially disconnectable structure and is cut off according to the print size . the connector 7 has a flexible cable 71 which connects to the printed circuit board 63 of the thermal head 6 , wherein a lead wire 73b which connects to the pattern 63b is grounded whereas a lead wire 73a which connects to the pattern 63a is connected to the input terminal a of the head discrimination circuit 10 . the head discrimination circuit 10 is composed of a latch circuit having the same signal level at the input a and output b thereof , and also has a function of preventing erroneous printer operation by the noise at the insertion of the thermal head 6 . fig3 shows the details of the selecting circuit shown in fig1 wherein shown are and gates 111 , 112 of transmitting either the signal from the position detector 3 or the signal from the position detector 4 in response to the signal from the head discrimination circuit 10 , an or gate 113 for transmitting the signal from the position detector 3 or 4 to the motor drive circuit 1 and the head drive circuit 12 , and an inverter 114 . fig4 shows the details of the head drive circuit 12 composed of a one - shot multivibrator 121 triggered by the signal from the selecting circuit 11 , and an and gate 122 , wherein the drive time of the print head is determined by said one - shot multivibrator 121 . fig5 shows another embodiment of the head drive circuit 12 in which the voltage supplied to the print head is regulated according to the print size . in fig5 there are shown a one - shot multivibrator 123 to be triggered by the output signal from the or gate 113 of the selecting circuit 11 ; and gates 124 , 125 to be controlled by the output signals from and and gates 111 , 112 ; amplifier 126 , 127 respectively providing signals for different voltages thereby applying a higher voltage in a larger print size ; and an adding circuit 128 for transmitting the output from the amplifiers 126 , 127 to an amplifier 12a . the above - explained thermal printer functions in the following manner . upon insertion of the thermal head 6 into the connector 7 of the carriage 5 , the input terminal a of the head discrimination circuit 10 connected to the power supply through a resistor r changes the potential level according to the kind of thermal head 6 inserted , since the pattern 63a on the printed circuit board 63 of the thermal head 6 is connected to said input terminal a of the head discrimination circuit 10 through said connector 7 , while the lead wire is grounded . thus , upon insertion of a thermal head 6 corresponding to a 12 - point print size in which the circuit pattern 63b is cut off , the input terminal a and the output terminal b of the head discrimination circuit 10 assume level &# 34 ; 1 &# 34 ;. on the other hand , upon insertion of a thermal head 6 corresponding to a 9 - point print size in which the circuit pattern 63b is not cut , the input terminal a and the output terminal b of said head discrimination circuit 10 assume level &# 34 ; 0 &# 34 ;. in response to the automatic discrmination of the thermal head 6 in this manner by the head discrimination circuit 10 , the selecting circuit 11 selects the position detector 3 or 4 for position detection for power supply to the thermal head 6 . upon receipt of a print instruction signal a state occurs where the carriage 5 is loaded with the thermal head 6 , the servo - motor 2 is activated through the motor drive circuit 1 and the amplifier 1a to displace the carriage 5 and the thermal head 6 in the print direction . in this state the output signal from the selecting circuit 11 is supplied as a feedback signal to the motor drive circuit 1 to control the servo - motor 2 at a constant speed corresponding to the 12 - point or 9 - point thermal head . when the position detection signal is transmitted from the selecting circuit to the head drive circuit 12 in the course of the carriage displacement , said head drive circuit 12 maintained in the stand - by state by the print instruction signal determines the head drive time and activates the heat elements 61a through the amplifier 12a , connector 7 and printed circuit board 63 at a defined position of the recording paper , thereby achieving thermal printing . in this manner the displacing speed of the carriage 5 is regulated according to the kind of the thermal head 6 , thus modifying the active position with respect to the recording paper 8 and forming proper characters . naturally such proper head displacement can be achieved by appropriate regulation of the carriage displacement speed or the timing of power supply to the head along . the present invention is not limited to the foregoing embodiment but is also applicable to a dot matrix thermal head for example 5 × 7 elements , or other printing heads such as a wire dot print head or a print head with printing types . furthermore , head discrimination is not limited to two different kinds but can be extended to three or more different kinds , and the discriminating can also be modified in various manners . also it is possible to obtain plural different prints from a same printing head if the discrimination is rendered manually selectable . as explained in the foregoing , the present invention allows to print different characters with exchangeable thermal print head by discriminating the kind of characters of said print head and accordingly controlling the speed and position of the print head , and is extremely in extending the range of printing with a simple structure .