A dynamo is an electrical generator. It works on the principle of using rotating coils to convert mechanical rotations into direct electric current. Dynamo generators were the first large-scale generators available to deliver large amounts of power to an industrial complex. It was the forerunner to present day electrical generators such as the electric motor and the alternator. Dynamos are obsolete today because of the widespread use of alternating current, the simplicity of converting alternating current to direct current and the proliferation of small solid state devices that require small scale, more efficient power sources. Regardless of the size and make-up, dynamo generators obey ohms law where current equals voltage divided by resistance.