Are you talking about the white dome shaped object? If so it's just a 25kv porcelain insulator. It's being used to support the phase (hot) conductor that serves your property. The oval shaped device on the crossarm is called a cutout. The skinning side of the cutout is the fuse barrel, which contains a one time use fuse. The single phase is tapped off the main 3 phase using a hot line tap, connects to the cutout, comes out the bottom of the cutout to the porcelain insulator and then over to the next pole towards your property. The wires below all that are neutrals. So the system that feeds you is a grounded-wye system. In my area of operation we run a 7,200/12,470 volt system. If you measure between the phases you with get 12,470 volts. If you measure from any of the single phases to the neutral you will 7,200 volt.