190 Telephone Systems of the Continent of Europe 



is looped into the cord. There is also a Morse key switch M, 

 having its lever connected through the cord to the tip (which is 

 insulated from the body) of the plug s 1 ; its back stop to a test 

 battery of one Daniell cell v 1 through an adjustable resistance G, 



FIG. 59 



and its bottom stop to earth through a i5o-ohm galvanoscope z. 

 Subscriber L 2 , in taking his receiver off its hook when making a 

 call, puts his test cell in connection with the line and immediately 

 blocks it against intrusion, since an operator testing by applying 

 the tip of plug s 1 to any of his jacks and pressing the key M 



