fig1 illustrates , in block diagram form , an example of an apparatus for the implementation of the method of the invention . illustrated in this example is an apparatus which is based on a known measuring installation for the implementaticn of a sampling method with the assistance of an electron probe . apparatus for the implementation of a sampling method with the assistance of an electron probe is known , for example , from the aforementioned u . s . pat . no . 4 , 220 , 854 . the implementation of a sampling method is also disclosed in this u . s . letters patent . a somewhat improved apparatus for the implementation of a sampling method is additionally known from u . s . pat . no . 4 , 227 , 679 , also fully incorporated herein by this reference . given such a known apparatus for the implementation of a sampling method , primary electrons pe are shaped by way of an electron beam blanking device bp to form sampling pulses ep ( fig2 ) via a sequence control con synchronous with the basic pulse rate cl ( fig2 ) applied to a unit under test pr . a cyclical signal si is applied to the measuring point on the unit under test pr . the cyclical signal si is documented in that secondary electrons se are triggered upon incidence of the primary electrons pe on the measuring point , the secondary electrons being documented in a detector dt and being converted into a secondary electron measuring signal . this measuring signal is first amplified in a preamplifier a1 . given the known sampling method , the signal si is sampled repeatedly in succession at one and the same phase point in order to obtain a good signal - to - noise ratio in the determination of the potential at the measuring point on the unit under test pr which appertains to a specific phase point of the signal si . the measuring signals which belong to one and the same phase point of the signal si are summed up in a circuit sh , for example a sample - and - hold circuit or an integrating circuit ( boxcar integrator ). the result of this summation is then again amplified in another amplifier a2 . the measured result for the potential which is applied to the measuring point on the unit under test pr at a specific phase point of the signal si is finally obtained at an output u of the second amplifier a2 . in order to linearize the measured result at the output u , the measuring chain comprising the detector dt , the amplifier a1 , an integrating circuit sh , and the amplifier a2 is closed to form a feedback loop fl which controls the voltage at a retarding grid gr in a retarding spectrometer for the determination of the secondary electron energy . for the implementation of the method of the invention , the measured signal is tapped at a point p between the preamplifier a1 and the integrating circuit sh and , for example , is supplied to a further amplifier a3 . the output of the further amplifier a3 is fed to the edge - controlled setting inputs of two rs flip - flops f1 and f2 . the output of the flip - flops f1 and f2 are combined in an and gate g . the output of the and gate g , finally , indicates whether a noise signal has or has not appeared in a region between two edges of the basic pulse rate cl of the unit under test pr . fig2 and 3 illustrate the function of a known apparatus according to the sampling method which does not permit a noise signal appearing between clock edges of the basic pulse rate cl to be documented . fig2 illustrates the basic pulse rate cl at the unit under test pr , the signal si which is applied to the measuring point in a cyclical fashion and which comprises a noise signal sp generated by the unit under test , the primary electron sampling pulses ep which appear at every edge of the basic pulse clock rate cl , the measured signal p1 at the measuring point p , and the measured result u1 at the output u . as the result u1 shows , the noise signal sp in the signal si cannot be documented with a known method and a known apparatus . as fig3 shows , the phases ph of the phase points at which the signal si is repeatedly sampled in succession over a finite time interval is shifted across the entire cycle of the cyclical signal si step - by - step given a known method . considerable spacings thereby respectively occur between the individual phases of various successive phase points , so that only a few phase points are covered given a registration of a cyclical signal si . fig4 and 5 show what improvements can be achieved by practicing the present invention and how such improvements can be obtained . fig4 shows a measuring signal p2 which can be tapped at the point p with an apparatus for implementing the method of the invention . given the method of the present invention , as shown in fig5 the phase ph at which the sampling pulses ep sample the cyclical signal si at the measuring point is not varied step - by - step in large steps as is shown in fig3 for the known method , but individual phase points of the signal si at which the phase ph remains constant over a longer time t are extracted , which means that these phase points are repeatedly sampled in succession in order to acquire a well - defined potential value which is exhibited by the measuring point at these phase points of the signal si . between two such phase points at which well - defined potential values u are acquired , all mensurationally - distinguishable phase points are only respectively sampled until a determination can be made as to whether a noise signal sp exists at these phase points . at these latter phase points of the cyclical signal si , therefore , sampling is in no way undertaken until , due to a good signal - to - noise ratio , a well - defined potential value can be specified at the output u , but only until it can be specified at the output s whether a noise signal exists in the intermediate region between two clock edges of the basic pulse rate cl . in practicing the present invention , therefore , the cyclical signal si is also sampled at such phase points at which the rise and decay of the noise signal sp can be identified . when a primary electron sampling pulse ep impinges the measuring point at the leading edge of the noise signal sp , then a corresponding , positive signal is generated therefor in the measured signal p2 given the method of the present invention . at the trailing edge of the noise signal sp , a negative signal is generated in the measured signal p2 in an analogous fashion . the measured signal p2 is amplified in the amplifier a3 and is supplied to the edge - controlled setting inputs of the flip - flops f1 and f2 . the flip - flop f1 responds to the leading edge cfthe noise signal sp and the flip - flop f2 responds to the trailing edge of the noise signal sp . the signal acquisition at the output u occurs at the same sampling times as in the known method of fig2 . the signal si is sampled according to the known sampling method at these sampling times at the edges of the basic pulse rate cl , i . e . a specific phase point of the signal si is repeatedly sampled in succession . when at least two signal changes occur in the measured signal p2 between two such sampling times which coincide with the edges of the basic pulse rate cl , then both flip - flops f1 and f2 are set . the output s then indicates that at least one noise signal has appeared between two such sampling times which coincide with the clock edges of the basic pulse rate cl . the sequence control con must reset the flip - flops f1 and f2 after at least every clock edge after which the phase point of the signal si to be sampled is to be changed . in order to obtain the measured result u2 , the noise signal indicator s must also be interrogated in addition to the signal at the output u , being interrogated after every clock edge of the basic pulse rate cl after which the phase point of the signal si is to be changed . intermediate regions in which at least one noise signal has appeared must then be separately flagged in the reproduction of the measured status diagram u2 . at least one noise signal has appeared in the shaded region given the measured result u2 in fig4 . although i have described my invention by reference to a particular illustrative embodiment thereof , many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention . i therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art .