The pilot stated that he selected gear down but did not check for a down and locked indication. After having kept sufficient power applied to land well down the runway, the pilot heard the gear warning horn immediately prior to touchdown, but could not prevent the aircraft landing with the gear retracted. When the pilot purchased the aircraft it had been fitted with an unguarded switch type circuit breaker adjacent to the gear selector handle, and a receptacle for a remote gear switch. These modifications, which had not been approved by the Department of Aviation, were apparently designed to allow the gear to be raised or lowered from outside the aircraft during maintenance checks when the aircraft was supported by jacks. When selecting the gear down on this occasion, the pilot had evidently knocked the circuit breaker to the "off" position, removing electrical power to the circuit. Neither the pilot nor the passenger, who held a current Commercial Pilot Licence, realised that there were no aural or aerodynamic indications associated with the extension of the gear into the airstream when the gear was selected down.