u per revolution of the main shaft 22 is equal to or possibly a multiple of said value a in order to achieve this the arithmetic unit 19 consists of a counter 23 and a buffer 24 whose operation is described in more detail further on the mechanism 20 which provides the signal 21 is formed by a pulse generator 25 which is mounted on the main shaft of the weaving machine and which generates a fixed number of pulses per revolution and a frequency multiplier 26 such that the signal 21 consists of a pulse train with a constant number of pulses per revolution of the main shaft 22 this number is henceforth referred to as z also shown schematically in fig1 is the main drive 27 of the weaving machine the counter 23 has two inputs 28 and 29 and is designed so that the values which appear at the two inputs are summed and the result r passed from the output 30 to the buffer 24 the logic of the counter 23 is such that each time the sum obtained in the counter 23 is greater or equal to the value z one pulse is supplied at the output u and in this case the sum present in the counter is reduced by z in other words if the value of the sum is exactly equal to the value z the result r 0 is obtained while if the sum is greater than the value z the result r supplied to the buffer 24 is equal to the remainder in the counter the buffer 24 passes the value r from its input to its output each time a pulse of the signal 21 is supplied to.