The earliest dynamos used permanent magnets to create the magnetic field. These were referred to as "magneto-electric machines" or magnetos. However, researchers found that stronger magnetic fields — and thus more power — could be produced by using electromagnets (field coils) on the stator. These were called "dynamo-electric machines" or dynamos. The field coils of the stator were originally "separately excited" by a separate, smaller, dynamo or magneto. An important development by Wilde and Siemens was the discovery (by 1866) that a dynamo could also bootstrap itself to be "self-excited", using current generated by the dynamo itself. This allowed the growth of a much more powerful field, thus far greater output power.