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A photograph taken in Colorado Springs, showing the general way in which Tesla sometimes obtained two oscillations.  The picture shows a large ball which formed the terminal of the transmitting circuit wound on a frame running around the laboratory, which is seen in the background, and which was used as one element of the complex transmitter.  As the other element he would use a second coil of which he had a considerable number, and one of which is shown in the photograph to the left of the observer, at a distance of something like 20 or 25 feet from the ball.  When employing the apparatus in accordance with the invention described in his METHOD OF SIGNALING application a coil, as the one illustrated, would be connected to a terminal, such as the ball shown, with one of its ends, while the other end was grounded or, as the case may be, connected to the ground through a condenser.  When the transmitter was set at work and the discharge took place, both of the systems would vibrate, each its own period, and thus two vibrations of suitable frequency were passed into the ground, and were used to excite two receiver circuits, each tuned to one of them.  This method was also used in obtaining more then two oscillations.