A copying machine admits a web of freshly exposed photographic paper directly into a cassette whose core engages and convolutes the web therearound. The progress of engagement between the leader of the web and the core is monitored by comparing the peripheral speed of the core with the peripheral speed of a rotor forming part of an advancing roll for the web and receiving torque from a variable-speed motor which also drives the core. The peripheral speed of the rotor increases when the leader of the web is adequately attached to the core, and this is detected by a circuit having first and second photoelectronic monitoring devices which respectively monitor the peripheral speeds of the rotor and the core. The signals which are generated by the monitoring devices are processed, and the processed signals are used to change the speed of the variable-speed motor, to deactivate a catcher mechanism which directs the leader of the web toward the core and assists in the establishment of adequate engagement between the leader and the core, to change the supply of energy to the motor as the diameter of convoluted web on the core increases, and/or to actuate an alarm device when the completion of attachment of the leader to the core is unduly delayed.