THE ELECTRIC TELEGRAPH. 



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on each side of the handle are two plates of metal, which may be called LP, 

 the left plate, and RP, the right plate. They are connected with the needle 

 wire, NW, which passes before and behind the magnetized needle, N, suspended 

 perpendicularly on its axis, with its north pole upwards. As long as the 

 wires, C\v and ZW, remain insulated from each other, no current passes from 

 the cell ; but as soon as the handle, H, is turned, so that the copper end 

 touches the left plate (fig. 2), and the zinc end touches the right, communi- 

 cation is established between the plates of the cell, and the current com- 

 mencing at the copper passes along cw, the top half of H, into LP, along 

 NW, travelling up before and down behind the needle, causing it to deflect to 

 the observer's left, according to the rule given above. Reaching RP, it 

 passes downwards to the cell to z, and so on to c, continuing its travels 

 and keeping the needle deflected as long as the handle remains in contact 

 with the plates. If it is required to deflect the needle to the right, the 

 handle is turned to the right, bringing its copper end in contact with the 



Fig 252.- Passage of the current (i). 



right plate, causing the current to travel in the opposite direction. By 

 following the current from the copper to the zinc, as indicated by the arrows 

 in fig. 3, it will be seen that it now travels up behind and down before the 

 needle, deflecting it to the observer's right. Thus, by causing a current 

 of voltaic electricity to pass alternately /// before and down behind, and up 

 behind and down before, the needle is moved to the left or the right at will. 

 The way in which the current is made to act on a distant needle is now simple. 

 The following figure (fig. 253) shows the arrangement. The left portion 

 of the figure represents an instrument at London, that on the right an instru- 

 ment at York. The needle-wire, instead of being continued directly to the 

 zinc plate of the battery, passes away from the needle over poles to York, 

 where it joins an instrument similar in all respects to that at London. It 

 passes similarly before and behind a magnetized needle, joining the right 

 plate of the instrument. As long as the two plates are unconnected, no 

 current can pass. The current is therefore completed by a contrivance 

 which may be represented by the semicircular piece of metal, K. In practice 



