according to fig1 , a supplementary device z comprises a voltage source u , which is connected to a first terminal 1 of the supplementary device z . the supplementary device z further comprises a microcontroller mc , which is connected to the output of a comparator k . a reference voltage produced by a resistor r is applied to the one input of the comparator k having its other input connected to a second terminal 2 of the supplementary device z . a small electric device e also has two terminals , which are connected by an electric lead l to the two terminals 1 , 2 of the supplementary device z . the small electric device e contains an internal resistor , which is schematically represented in fig1 by a resistor ri connected between the terminals of the small electric device e . in this embodiment the supplementary device z continuously measures the resistance across its terminals 1 , 2 , which is practically infinite in magnitude when no small electric device e is connected . if , on the other hand , a small electric device e is connected to the terminals 1 , 2 of the supplementary device z , an electric current will flow through the internal resistor of the small electric device e on account of the voltage supplied by the voltage source u , resulting in a corresponding voltage at the second terminal 2 of the supplementary device z , which is compared by the comparator k with the reference voltage . if the internal resistance of the small electric device e is small enough , the output of the comparator changes from “ low ” to “ high ” or vice versa . this enables the microcontroller mc to detect that the supplementary device z is connected to a small electric device e . in other embodiments there are more than two electric terminals , of which two are used , for example , for the supply of electricity to the small electric device and a third is provided specially to detect the connection and / or the type . for this purpose the third terminal is connected , for example , to a resistor referred to as a pull - up or pull - down resistor , whose resistance varies according to the type of small electric device . the supplementary device is then able to identify the type of small electric device from the measured resistance or from the voltage measured at the third terminal . instead of an electric connection it is also possible to provide an inductive coupling between , for example , a charging device and a small electric device . the supplementary device z shown in simplified form in fig2 , 3 and 4 has three terminals . the first terminal 1 is connected via a first switch s 1 to a voltage source symbolized by an arrow , which is connected in turn to the one end of a first resistor r 1 . the first resistor r 1 has its other end connected to the second terminal 2 . between the second and the third terminal is a second switch s 2 . both switches s 1 , s 2 are controllable by a microcontroller mc . the third terminal 3 is connected to ground . the second terminal 2 is connected furthermore to the one end of a second resistor r 2 . the second resistor r 2 has its other end connected to the cathode of a zener diode zd . the zener diode zd has its anode connected to a first input of a comparator k and to the one end of a third resistor r 3 . the other end of the third resistor r 3 is connected to ground . the output of the comparator k is connected to the microcontroller mc . the comparator k has a second input , not shown in fig2 , 3 and 4 , which is connected to a reference voltage source which likewise is not shown . as an embodiment for a small electric device fig2 shows an electric shaving apparatus e 1 containing a storage battery a , whereas fig3 shows an electric shaving apparatus e 2 containing no storage battery . both shaving apparatuses have three terminals each , which are electrically connected to the corresponding terminals 1 , 2 , 3 of the supplementary device z . furthermore , both shaving apparatuses have a motor m and a series circuit comprised of a fourth resistor r 4 and a light - emitting diode ld . in the shaving apparatus e 1 shown in fig2 there is a series circuit comprised of the storage battery a and the motor m positioned between the second and the third terminal , while the series circuit comprised of the fourth resistor r 4 and the light - emitting diode ld is positioned between the first and the second terminal . in the shaving apparatus e 2 shown in fig3 the motor m is positioned between the first and the second terminal , and the series circuit comprised of the fourth resistor r 4 and the light - emitting diode ld is positioned between the first and the third terminal . a method of the present invention by which the cleaning device can identify whether a shaving apparatus is connected to the terminals and , if so , which type is involved , will be explained in the following with reference to fig2 and 3 . the steps required to perform this method are controlled by the microcontroller mc . in an initial position , in which the cleaning device is not connected to a shaving apparatus , the two switches s 1 , s 2 are not closed . the zener diode zd is conducting , and at the first input of the comparator k is a voltage determined by the voltage of the voltage source and the resistances of the first , second and third resistors r 1 , r 2 , r 3 . said resistances and the voltage of the reference voltage source are of a magnitude causing the output of the comparator k to be “ high ”. if a shaving apparatus is now connected to the cleaning device , the voltage residing at the second terminal 2 will now be lower , corresponding in the case of a shaver with a storage battery a to approximately the voltage of the storage battery and being determined in the case of a shaver without a storage battery by the resistances of the light - emitting diode ld and the first and fourth resistors r 1 , r 4 . the zener diode zd is non conducting , therefore , and the third resistor r 3 connects the first input of the comparator k to ground , whereupon the output of the comparator k becomes “ low ”. the microcomputer mc can identify from the “ low ” potential that a shaver is connected to the cleaning device . in order to then determine the type of shaver connected , the first switch s 1 is closed temporarily by the microcontroller mc . in the case of a shaver with a storage battery a the voltage at the second terminal 2 does not change as the result but continues to be determined by the voltage of the storage battery a . the output of the comparator k remains accordingly at “ low ”. in the case of a shaver without a storage battery , on the other hand , the motor m of the shaver is connected in parallel to the first resistor r 1 as the result , causing the voltage at the second terminal 2 to increase temporarily to the extent that the zener diode is temporarily placed in conduction and the output of the comparator k becomes “ high ” temporarily . hence , if the output of the comparator k also changes over temporarily to “ high ” when the first switch s 1 is closed temporarily , the microcontroller mc knows that a shaver without a storage battery is connected . by contrast , if the output of the comparator k remains at “ low ”, a shaver with a storage battery a is connected . if the motor m of the shaver is to be switched on in the course of a cleaning operation , the second switch s 2 , will be closed by the microcontroller mc . when a shaver with a storage battery a is connected , the motor m is supplied with electricity by the storage battery a . when the connected shaver has no storage battery , the motor m is supplied with electricity , by the cleaning device . in a preferred embodiment shown in fig4 , the supplementary device and the small electric device include an arrangement for the transmission of data as described , for example , in de 198 17 273 initially referred to , the contents of which are hereby incorporated in the subject - matter of the present application by express reference . data is transmitted via the first terminal 1 . a shaving apparatus e 3 containing a storage battery a and a microcontroller c required for the data transmission is shown in fig4 as an embodiment for a small electric device . the shaving apparatus also has a third switch s 3 which is controllable by the microcontroller c . to charge the storage battery a the microcontroller c of the shaving apparatus e 3 closes the third switch s 3 , and the microcontroller mc of the supplementary device z closes the first switch s 1 , whereby a charging circuitry , not shown in fig4 , in the shaving apparatus is connected via the first terminal 1 to the voltage source of the supplementary device . the supplementary device thus serves also as a charging device for the storage battery a in the shaving apparatus . according to the present invention , data transmission is effected by the supplementary device feeding an electric current , an electric voltage or an electric and / or magnetic field to the small electric device , which current / voltage / field is modulated with a corresponding data signal by the small electric device or by the supplementary device , depending on the required direction of data transmission . demodulation takes place in the respective receiving device . one method which is particularly easy to implement performs the modulation by switching on and off the electric current , the voltage or the field . if , with the devices shown in fig4 , data is to be transmitted from the supplementary device z to the small electric device e 3 , the microcontroller mc in the supplementary device generates a data signal driving the first switch s 1 , causing it to be opened and closed accordingly so that the voltage residing at the first terminal 1 is amplitude - modulated by the on / off switching operation . at the same time the microcontroller c of the small electric device e 3 detects , with the aid of a sensor such as a comparator , that the voltage is modulated , demodulating the received data . data transmission in reverse direction , that is , from the small electric device to the supplementary device , can be performed in similar manner . by transmitting suitable control signals the devices are thus able to set the switches in the required position for charging the storage battery a . once the storage battery is sufficiently charged , the shaver can transmit a corresponding data signal to the supplementary device , whereby the first switch s 1 is opened , terminating the charging operation .