instrument using magnets to detect presence of iron
• (n.) An instrument for detecting small quantities of iron in any substance by means of a very delicate combination of magnetic needles.
Sid'er·o·scope noun [ Greek ......... iron + -scope .] An instrument for detecting small quantities of iron in any substance by means of a very delicate combination of magnetic needles. 
sideroscope A magnet or other appliance for determining the presence of metallic iron as a foreign body in the eye.