Mobile crane generally operate a boom from the end of which a hook is suspended by wire rope and sheaves. The wire ropes are operated by whatever prime movers the designers have available, operating through a variety of transmissions. Steam engines, electric motors, and internal combustion engines (IC) have all been used. Older cranes' transmissions tended to be clutches. This was later modified when using IC engines to match the steam engines' "max torque at zero speed" characteristic by the addition of a hydrokinetic element culminating in controlled torque converters. The operational advantages of this arrangement can now be achieved by electronic control of hydrostatic drives, which for size and other considerations is becoming standard. Some examples of this type of crane can be converted to a demolition crane by adding a demolition ball, or to an earth mover by adding a clamshell bucket or a dragline and scoop, although design details can limit their effectiveness.