THE ELECTRIC TELEGRAPH. 



137. To explain the manner in which the deflection of the 

 needle depends on the direction of the current, let us suppose 

 the needle to be placed on an horizontal axis o, fig. 54, so as to 

 Fig 54. play in a vertical plane, and to be 



maintained in the vertical direction 

 when not affected by the current, by 

 giving a slight preponderance to the 

 arm on which the south pole of the 

 needle is placed. By this arrangement 

 the needle, when undisturbed, will rest 

 in the vertical position, the north pole 

 IT being directed upwards, and the 

 south pole s being directed downwards. 

 Now if the current which is before 

 the needle be directed downwards and 

 that which is behind it upwards, the 

 north pole IT will be deflected to the 

 right, and consequently the south pole s to the left, as represented 

 in the figure. But if the direction of the current be reversed so 

 that before the needle, it shall be directed upwards and behind 

 it downwards, the north pole N will be deflected to the left and 

 the south pole s to the right. 



If the intensity of the current be great, and the preponderance 

 given to the lower arm of the needle small, the deflective force of 

 the current will be sufficient to throw the needle completely at 

 right angles to its position of rest, that is, to give it the horizontal 

 direction ; but it is important to observe, that no greater intensity 

 of the current can affect it further. The north pole, for example, 

 cannot be deflected downwards, or the south pole upwards. In 

 fine, the needle cannot be more affected by any increase of force 

 of the current after it has once been thrown into the horizontal 

 direction. 



If the intensity of the current be insufficient to throw the needle 

 into the horizontal direction, it will nevertheless take a position 

 intermediate between that and the vertical direction at which it 

 will rest. Its deflection from the vertical will be more and more 

 considerable as the current is more intense, and certain mathe- 

 matical conditions have been discovered by which the relative 

 intensity of the current may be determined by the amount of 

 the deflection of the needle which it produces. 



138. It is evident that the sensibility of the needle will be so 

 much the greater as the preponderance of the arm s is diminished 

 and the intensity of the current increased. An expedient has, 

 however, been ingeniously contrived, by which the most feeble 

 current can be made to affect the needle. This is accomplished by 

 194 



