and 3 is guided by a wheel 48 heated by means of a heat source not shown the wheel 48 is driven in a rotating manner as illustrated in fig2 by the arrow 50 and may also be provided with a drive again the direction of travel of the object to be measured is inconsequential the rotational direction shown in fig2 is hence merely an example and the invention can also be applied in the opposite direction when the conductor enters from the right in fig2 to guide the metal wire 10 the wheel 48 as can be seen in fig3 has a groove 52 with a v shaped cross section that is formed along its running surface 54 it can also be useful to loop the metal wire 10 shown in fig2 around the entire perimeter of the wheel 48 the surface of the groove 52 and possibly also the running surface 54 can be provided with a black coating to again largely approximate a black radiator furthermore it can also be seen in fig2 and 3 that an additional radiation source is provided beyond the heated wheel 48 as the first radiation source that is a radiation source 56 with a u shaped cross section that sectionally covers the wheel 48 and especially its groove 52 see fig3.