end with a recess extending around the pipe inwardly from the pin or box end each recess contains an insulating sleeve 21 mounting the contacts 17 to allow electrical communication to be made with like contacts and conductors of adjacent pipe units the two insulating sleeves 21 and the contacts 17 have outer surfaces which are continuous with the adjacent outer wall surface of the pipe units between them the contacts 17 can have any appropriate external shape for example as indicated at inset a of fig5 the contacts can comprise slip rings 17a extending entirely around the unit alternatively as shown in inset b of fig5 they can comprise segmented slip rings 17b at three axially spaced positions another possibility namely segmented slip rings 17c at the same axial position is illustrated in inset c of fig5 in the mechanically assembled condition of two pipe units 1 shown in fig5 the two units are rigidly connected together mechanically and their communicating interiors provide a flow conduit for drilling mud it remains to establish electrical connection between the two sets of the conductors 10 this is effected by means of bridging connector means constituted here by bridging members 25 axially split into sleeve portions 26 shown in fig3 each provided internally with electrical contact bands 27 appropriate to the pattern of the contacts 17 in the contact zones 19 the connector sleeve.