An algorithm is proposed to center the beam in the quadrupoles of a transfer line, by changing the focussing st,rength of a quadrupole and observing the resulting position shift on a monitor downstream. The observed position shift depends linearly on the beam position offset in the quadrupole, provided the phase advance between the quadrupole and the monitor is not a multiple of r. The same monitor may thus be used to center the beam in several places of the beam line. The centering accuracy

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... ends on the resolution of the monitor and on the quadrupole current increment which can be set in such a way that the beam profile remains sufficiently peaked to determine its mean position. I.