He was commissioned as an ensign in the United States Navy. During his ensuing naval service, he first served on the USS "Richmond", then the USS "Minnesota". While in Asia, Sprague wrote stories he filed for the Boston Herald. While his ship was in Newport, Rhode Island, in 1881, Sprague invented the inverted type of dynamo. After he was transferred to the USS "Lancaster", the flagship of the European Squadron, he installed the first electric call-bell system on a United States Navy ship. Sprague took leave to attend the International Exposition of Electricity of 1881 in Paris and the Crystal Palace Exhibition in Sydenham, England in 1882, where he was on the jury of awards for gas engines, dynamos and lamps.