You might check here. https://www.google.com/search?source=hp&ei=TjXNXtXAOLfI0PEP6JeIwA4&q=about+metal+detectors&oq=about+metal+detectors&gs_lcp=CgZwc3ktYWIQDDICCAAyBggAEBYQHjoFCAAQgwE6CAgAEBYQChAeUNoUWKxtYM6eAWgAcAB4AIABZ4gB_AySAQQyMC4xmAEAoAEBqgEHZ3dzLXdpeg&sclient=psy-ab&ved=0ahUKEwjVn67V69HpAhU3JDQIHegLAugQ4dUDCAk