Counting experiments were made with cylindrical counters fitted with geometry ``definers'' in order to ascertain whether it is possible to find an area of constant sensitivity in these counters. These measurements are similar to those made by Gleasonet al.with end‐window counters. It is possible to achieve this result by means of a careful choice of the dimensions of these definers. The definers also allow a great simplification of the calculation of counter efficiency for a point source. We present a formula for sources placed in any position with respect to the counter. This formula is compared with experiment and the agreement is found to be good. A comparison of absolute beta counting between one of our cylindrical counters (with definer) and an end‐window counter (with definer) is made. The agreement obtained is satisfactory.