I2O Telephone Systems of the Continent of Europe 



the subscriber's plug with flexible cord ; /j and p* are the head 

 and tube of the plug respectively, pc is the subscriber's plug 

 contact, cutting out the relay sr, when the plug sp is removed, 

 and at the same moment joining the battery to the test wire /, so 

 causing the subscriber to test ' busy.' sk is the line key. When 

 this is pressed down, the operator's telephone apparatus is cut in 

 between /j and / 2 . sk and sp are for convenience placed in close 

 proximity. The operator's apparatus consists of kp and >/, keys 

 for speaking to plug or to line respectively. These keys are not 

 used under normal conditions, and only when it is necessary to 

 speak to one side, insulating at the same time the other side, cp 

 and cl are calling keys for effecting the ringing to plug or to line 

 respectively. When one is depressed, it is at the same time 

 possible to speak to the other side. Under normal conditions 

 only cp is used, g is the generator for the ringing current. / and 

 mb the microphone and telephone combined into a microtelephone 

 set, suspended and balanced by a counterpoise. The apparatus 

 is connected by a flexible cord to a four-way plug and jack (only 

 shown in the figure by dots), so that a new microtelephone set 

 may be immediately inserted, me is the microphone battery 

 contact, and tc the test contact, giving a road to earth through a 

 self-induction coil (not shown in the diagram) for the test current 

 when this has passed the telephone. These contacts, me and tc t 

 may be left out if another four-way plug, connected to a second 

 microtelephone set without such contacts, or to a head telephone 

 and microphone, be inserted. The connecting wire between tc 

 and ground might then be connected to a wire in the telephone 

 between the magnet spools, the test current going in this 

 manner through only one of the coils, which is quite sufficient for 

 the testing. 



The mode of operating is as follows : When sd falls, sk is 

 depressed and sp lifted by practically the same movement with the 

 right hand, while the number of the wanted subscriber is received 

 through the telephone. After testing by touching the jack of the 

 line called for with the plug head and pressing at the same time 

 tc with the left hand, the plug is inserted in the jack and cp is 

 depressed for calling. When the connection is through, sk is 

 released and sd replaced. When sd falls again, in response to 



