RMS Tahiti: set a distance record of 5,500 miles with Estevan.

    Obviously in the normal course of station operation there must be some conveninet way of moving the antenna between the receiver and transmitter.

     In the photograph this change over is accomplished by rotating the gray handle 90 degrees clockwise thus rotating the long steel arm up into the vertical receive postition.  To receive, the arm was rotated into the horizontal position. The switch is shown in the transmit positon. 

Marconi Type 1 Change Over Switch

    In the down (TX) position the copper tip of the arm places a short across the two contacts on the base.  These contacts are in series with the AC generator rotor windings feeding the step up transformer's primary.  If  the contacts are not closed, ie arm not fully horizontal, there will be no AC voltage for the high voltage step-up transformer.

     In the up (RX) position the arm spreads the two copper fingers clasping the left stud.  This breaks the antenna connection to the transmitter.  (Note the right stud goes to the antenna, the left stud goes to the transmitter's coupling coil.)  Thus the transmitter tuned circuits will not interfere with the receiver.
     The drum of the switch has a number of brass tabs all insulated from its neighbour.  There is one on the long arm which, in the up position, connects the receiver to the antenna via the spread finger contacts and to the antenna stud on the right.  The other contacts are used to protect the operators ears by shorting the headphones out and another to short the receiver's crystal detector.
Drawings are from "Practical Wireless Telegraphy" Elmer Bucher 1918 edition.
Change over switch.
The image above from the Bowerman collection shows an older model of an antenna change over switch in the center of the photo.  The handle would be pulled down to the horizontal to change over.