The next important step in the progress of telegraphic invention, after that of Mr. Soemmering, was made by Mr. Ronalds, who in 1816 succeeded in making a perfect apparatus, that transmitted every requisite signal with the use of only a single circuit. In the agent employed, however, there was a retrogression to frictional electricity and the pith-ball electrometer, for at that time the property which a voltaic current possesses of deflecting a magnetic needle had not been discovered. Mr. Ronalds's plan was to have, at each communicating station, a good clock with a light paper disc fixed on to the seconds wheel, on which were marked all the letters of the alphabet, and the ten numerals. Only so much of this disc was exposed to view as to show a single letter at a time, through a small aperture, as the seconds wheel revolved. The clocks at the corresponding stations were set exactly together, so that the same letter was exposed to view at each instrument at the same instant. A pith-ball electrometer, connected in a single circuit with the transmitting station, was kept distended during the transmission of a message by charging the wire from an electrical machine; and when the letter required to be indicated appeared at the aperture of both instruments, the operator at the transmitting instrument instantly discharged the electricity of the wire by touching it, and thus caused the pith-balls to collapse. In this manner the person at the receiving station, by attentively watching the pith-balls, and noticing the letter that appeared at the instant of collapse, could read the messages signalled.