the stroboscope led observed on an oscilloscope by adjusting the position of the supply pulse from the led relative to the control of the actuating needle 1 it is then possible to draw the graph in fig5 where the motion of needle 1 is shown by the continuous curve and the motion of the latch is shown by the broken line curve elements a and b represent the control pulses as can be seen the speed of needle 1 at the moment of impact is of the order of 1 5 m s whereas the speed of the latch immediately after impact is of the order of 1 m s thus exceeding the previously calculated necessary speed which was 0 6 m s note that these tests were made under laboratory conditions on a single selection latch 2 always acting on the same butt of the latch which was returned by a spring acting on the other butt thus slowing down the butt compared with actual conditions which are therefore more advantageous however we chose a spring having very low stiffness so as to reduce perturbation to a minimum the repetition frequency of actuation can thus remain sufficiently high to reproduce operating conditions near those actually occurring fig4 illustrates the relative positions of needle 1 and latch 2 in their two respective end positions before and after the impact at rest latch 2 is 0 2 mm from the end of needle 1 after the impact the needle continues to move for 0 4 mm from its point of rest and the latch pivots into recess 6 reaching an amplitude.