ABSTRACT
Can you see the weight of a cable? This question, a paraphrase of the title of Kac's famous (1966) paper will be considered in Sec. 4. There are many more examples from physical systems where the source term of a differential equation is to be recovered like, for instance, inverse heat conduction, Mair and Ruymgaart (1996). These examples fit within this framework but cannot be considered here.
