or the side 53 has a channel 60 formed in it the channel 60 is defined in part by a c shaped collar 61 an inner wall 62 of the collar 61 forms a narrowed mouth portion 63 and a closed bottomed enlarged portion 64 the mouth 63 is sized to receive a cable 65 in a friction fit and to hold it in place to prevent movement of the cable 65 as is best seen in fig5 as may be observed the channel 60 is shown formed in the side 54 in fig4 and in the side 53 in fig5 either arrangement works well as indicated above the enclosure 51 is open along the top 52 the lower side 57 of the enclosure 51 delimits a mounting means 66 for reception of the hall effect sensors that is to say the mounting means 66 as best seen in fig5 delimits a shoulder 70 and an extension 71 the extension 71 has a predetermined thickness 72 within the thickness 72 is formed at least one pocket 73 sized for reception of a respective hall effect sensor 75 the preferred embodiment as illustrated employs three pockets 73 and three of the sensors 75 shown positioned in the enclosure 51 in fig5 the sensors 75 have a plurality of electrical connections 76 associated with them the connections 76 are designed to mate and electrically connect as necessary with the cable 65 along a circuit board 79 the circuit board 79 acts to connect the sensors 75 to their associated leads in the cable 65 in some predetermined arrangement the enclosure 51 includes an attachment means 80 for mounting the assembly 50.