Abstract: The wrapped cigarette rod which is formed in a cigarette making machine is monitored by a beta ray detector which generates first signals denoting the density of successive unit lengths of the moving rod. The first signals are compared with a reference signal which denotes the desired density of the rod, and the resulting third signals are squared, N squared signals are totalized and the resulting fourth signals are divided by signals denoting N - 1 first signals to form sixth signals. The square roots of sixth signals constitute regulating or adjusting signals which enable the attendants to feed tobacco or other fibrous material at an optimum rate. The regulating signals are converted into visible signals. The first signals can be used to adjust the position of the device which removes the surplus from a continuous tobacco stream to form a filler which is thereupon wrapped to constitute the cigarette rod.