points 1a and 1b after cable 2 has been fitted parallel to cable 1 and series impedances s1 and s2 have been switched to the state of low impedance value situation corresponding to fig2 d this procedure requires engineers working simultaneously at points 1a and 1b who via the necessary communications equipment synchronize cutting through as far as possible since cutting through is done by hand such synchronization can never be perfect and it will therefore be possible for interference nevertheless to arise in the time which elapses between cutting through at point 1a and at point 1b as series impedances s3 and s4 can be switched simultaneously via electronic or radiographic means the use of such means is to be preferred in principle it is conceivable that the series impedances s1 and s2 consist of switches which initially are open state 34 1 34 and later fig2 d are closed state 34 0 34 and are then kept definitively closed it is conceivable to realise an embodiment of this type using existing semiconductor techniques however this is expensive because the switches can subsequently no longer easily be removed from cable 2 furthermore such switches can certainly not be used for series impedances s3 and s4 because they would already necessitate the cutting through of cable 1 it is therefore desirable to be able to perform the method described above using means which can be fitted in a non destructive manner to a predetermined point in a cable and there are able temporarily to induce as desired a series impedance with variable impedance value in the cable and which can be removed on completion of.