A needle assembly comprises a metal needle with a hub and a marker in the form of a bubble-filled plastics rod inserted within the needle. The use of color-coded gowns follows the patient on his or her person and is therefore less likely to be lost or unseen information, thereby avoiding physical calamity and legal liability. The invention includes a weak magnetic cover which is located on the rolling bearing unit at a distance from the angle transmitter in such a way that a magnetic shield is formed between the angle transmitter and the environment outside. User signal processing units correct outputs from the antenna radio receiving units based on the phaseamplitude information.