1296606 Spark-erosion G G SEMIN 10 Dec 1970 58797/7 Heading B3V The rotation of a reel 13 from which a spark erosion electrode wire or band 2 is unwound, is stopped if the electrode 2 breaks during operation. During normal operation of the machine, the electrode wire 2 is unwound from the supply reel 13 by a reel 23 driven by a motor 22 which is more powerful than the motor 15 for the reel 13 so as to overcome the tendency for the motor 15 to rotate the reel 13 anticlockwise. If the wire 2 breaks, the driving shaft 14 of the reel 13 can rotate anticlockwise causing a ball 21 to be wedged between the shaft 14 and a surrounding sleeve 17. The sleeve 17 is thereby rotated with the shaft 14 until a projection 18 engages a fixed abutment 19 which prevents further rotation of the reel 13. An electrical circuit is completed when a contact 20 on the projection 18 engages the abutment 19 causing a motor 7 driving the workpiece table 5 to be stopped and the electrode power supply 1 to be disconnected.