yellow thermochromic colour pigments which turn transparent at a colour change temperature of 60 degrees celsius thus the cover layer 11 provided with the thermochromic colour pigments appears at room temperature in a green colour tone in operation the podiatrist removes the abrasive head 1 from a sterile packaging and clamps this by means of the shaft 5 into a chuck of the driving device during the grinding process the podiatrist presses the abrasive head 1 for example against a hardened skin area of the patient 39 s foot because of the friction between the abrasive layer 8 and the foot of the patient frictional heat is produced which leads to an increase of the outer face temperature of the abrasive cap 6 the cover layer 11 which in the cold condition at room temperature is still green turns yellow starting from an outer face temperature of the abrasive cap 6 of 40 degrees celsius thus the podiatrist recognises that the outer face temperature of the abrasive cap 6 is at a temperature above 40 degrees celsius which is pleasant for the patient but below 60 degrees celsius if the podiatrist does not remove the abrasive body 1 the outer face temperature of the abrasive cap 6 increases further with longer grinding time if the outer face temperature exceeds 60 degrees celsius the cover layer 11 turns transparent and the red base layer 10 becomes visible thus it is signalled to the podiatrist that the outer face temperature has.