Abstract: The amount of textile warp size added to a sheet of threads in a running slasher is controlled by measuring the force applied to a separator bar, by passage of alternate threads on alternate sides of the bar. This force is the result of the dried size, which causes adjacent threads to adhere to each other. The force is compared to a predetermined value corresponding to the desired amount of size to be added to the threads. Based on this comparison, the pressure on the nip rolls is adjusted in order to squeeze more or less of the size solution from the threads. In this way, the amount of size added is maintained substantially equal to the predetermined value.