THE SWITCH-BOARD 45 



The whole arrangement of Professor Korn's 

 selenium instruments has now been described, and 

 there remains only the description of the actual 

 commercial working to be given. It is clear 

 that there must be some means of communication 

 between the operators of the sending and receiving 

 machines . As a metallic circuit is necessary owing 

 to the weakness of the current transmitted, tele- 

 phone lines are employed, and these must be quite 

 isolated and free from annunciators or shunts. A 

 convenient arrangement is made by which the 

 operator can switch at will the line into his instru- 

 ment or into the telephone. The switchboard is 

 shown in Fig. 21 . 



There are two change-over telephone switches 

 on the board as shown. The change-over contacts 

 of one are connected to two terminals AA, which 

 lead to the receiving machine, denoted by G for 

 convenience. One of these lines passes first through 

 the milliampere metre MA to indicate the strength 

 of the line current. The change-over contacts of 

 the other switch go to the telephone line. It will 

 be seen that when the switches are in the position 

 shown the telephone line goes direct to the tele- 

 phone T, and the machine terminals are in connec- 

 tion with a 1,000 <Q resistance R. When switched 

 over towards the right the machine is in connection 

 with the telephone line, and the telephone instru- 

 ment is out of circuit. The three terminals C give 



