36o NINETEENTH CENTURY. pt. hi. 



4. The commutator^ b, is a box with an apparatus inside 

 which is so arranged that by turning a handle (not shown 

 in the diagram) different ways the earth wire and telegraph 

 wire can be joined together, or either of them can be joined 

 to one of the poles of the battery. 



The commutator and galvanometer are really made in 

 one instrument, but I have drawn them separate to make it 

 more clear. 



Now, when the man in London wants to send his message 

 to York, he first sends off a current which rings a little bell 

 at all the stations along the line to call attention, and then 

 spells out the word York. This warns the man at that 

 station to turn the handle of his commutator, b', so that the 

 telegraph-wire, d^ and the earth- wire, ^, are joined together. 

 Then the message can be sent. The man in London turns 

 his handle according as he wishes the current to go. In 

 Fig. 60 he has turned it so that the telegraph wire, c, is 

 joined to the positive pole of the battery, and the current 

 will pass above ground along ^ ^ to the galvanometer a', 

 turning the needle to the right, and will then go back through 

 the earth hy g f e to the battery. But in Fig. 61 the man 

 has altered the handle, and now the earth wire, <?, is joined 

 to the positive pole, and so the current passes underground 

 Zii e f, and out at g, and entering the galvanometer on thj 

 left side, turns the needle to the left, and goes back by tl 

 telegraph wire, d <r, to the battery. In this way he turns 

 from right to left as he will, and spells out the message thus 

 Left, right J = A; left, right, left, left, //// = L; left, rigl 

 right ,// = W ; left, \ = E ; therefore J ; J, ; J, ; J/| 

 w JuJ Ju; spells ' aU we//: 



It is not necessary to have a separate wire for every teli 

 graphic station : one wire will do all the work so long as it 



