Rav Goren has a questionable קולא About Metal Watches:
We generally assume like the רמב"ם (not like the ראב"ד) that מוסיף טומאהעל טומאתו only means that a כהן who is physically touching one thing that is טמא מת may touch another thing that is טמא מת , against רבינו תם and the ראב"ד . We Also hold that מעיקר הדין a piece of metal that was in contact with טומאת מת becomes טומאת מת itself, even though for some reason the רמ"א was מקיל with regard to such metal. Based on these two ,דינים Rav Goren claimed that if a כהן wants to come into contact with a מת , all he has to do is put on a watch that is טמא מת , which the רמ"א says is מותר , and then even according to the רמב"ם one could come into contact with a מת.
This is highly questionable. First of all, the רמב"ם may have only meant that there is no אסור דאורייתא for a כהן in contact with one מת to touch another, not that it is completely מותר. Also, the דין of מוסיף טומאה על טומאתו is learned from the word " להחלו ," which is interpreted as excluding anyone who is already מחולל ועומד . One could argue that a person is only considered מחולל ועומד when he is in contact with something that is אסור for him to come into contact with or something that is actually מטמא him. Only then is there already a חלל that coming into contact with another מת cannot worsen. It seems like somewhat of a תרתי דסתרי to paskin that one can touch the watch and then consider himself מחולל ועומד after doing so.