He then made drawings of the closed rail circuit system substantially as it is used today, and in 1871 applied for a patent thereon, broadly covering the closed rail circuit system. In 1872 he made an exhibition of this system at the State Fair, held at Erie, Pa. Here he placed a large gong on the end of one of the buildings, on the outside, and inside he had a track made in sections placed in a long water tank made for the purpose. The track was covered several inches deep with water and the running gear of the car model was similarly immersed. The system was connected on the short circuit principle through the rails. Wires connected the gong with the back contact of the track relay. The water had no perceptible effect on the operation of the apparatus, and when the car was run on the signal section it short circuited the current from the relay, which, releasing its armature, closed circuit through its back contact and thus through the magnet of the gong circuit, thus setting the gong ringing loud enough to be heard all over the grounds. On running the car off the section the current returned to the relay energizing the same and thus opening the gong circuit at the back contact of the relay, thereby causing the gong to cease ringing. The whole operation was perfect, demonstrating the successful operation of the closed circuit system, and attracted great crowds of people as well as the marked attention of practical railroad men. It will be understood, of course, that the local circuit may be normally open as above described and used, or normally closed as now commonly used, according to the exigencies or requirements, or preferences of the parties using the same, and when desired, a visual signal may be substituted for the audible signal above described. These are all minor details not involving separate invention. Mr. Robinson had previously explained the new closed rail circuit system to Mr. Baldwin, who was greatly interested and expressed his confidence in it and requested Mr. Robinson to install the system at Kinzua, where he had already installed the open circuit wire system. [Illustration: Fig. 1. Robinson's Closed Rail Circuit System. Philadelphia & Erie Railroad, 1872.] As all the signal apparatus, relays, batteries, office switches and overlapping devices were already in operation there, it took but a short time to convert this open circuit system into a closed rail circuit system.