Without looking it up, 4.5 volts. Originally there was an open-topped aluminium box that held 3 'X' or 'S' cells and slid into the brackets o the floor of the case, but I suspect they started using 126 or similar batteries at some point.
I have the manual and will look it up in a moment.
There should be slip-on weights with built-in pulleys for each of the connecting cords, and you alternate red and black cords to reduce confusion.
Chris.