132 THE ELECTRIC TELEGRAPH. 



tions of thick copper wire, forming the primary, and by a 

 long fine wire outside this, forming the secondary coil. The 

 primary coil is put in circuit with the key and with a battery 

 of large surface and little internal resistance. The secondary 

 coil is connected at one end with the earth, at the other with 

 the line. It is sometimes divided into two parts, which may 

 be connected parallel or in series, according to the resistance 

 of the line. 



82. The polarised relay differs in its construction from all 

 the others. There is no spring employed to pull back the 

 armature after it is let go by the poles of the electro- 

 magnet. 



Fig. 72 is a sectional view in the direction of the armature, 

 and Fig. 73 a top view of the relay. The perpendicular 

 electro-magnet E is composed of two cores of soft iron united 

 below, in the ordinary manner, by a cross-bar, A, also of 

 soft-iron. The coils of wire terminate at the screws 1 and 2. 

 The north end N of an angular bent permanent magnet, N s, 

 is screwed on to the cross-bar A, to which it communicates 

 north polarity beyond the point of contact, and also to both 

 the cores and poles of the electro -magnet E. The soft iron 

 tongue c is supported on an axis in a slit in the south end 

 s of the permanent magnet, and thus receives south polarity. 

 This tongue is so placed that it may oscillate between the 

 north poles N and N' of the electro-magnet. Its play is 

 limited by the contacts D and D f . D is used as a contact for 

 closing the local circuit, in which are included the printing 

 instrument and the local battery, when the tongue c strikes 

 against it. D' is furnished with an agate point, and while 

 the tongue rests against it, the local circuit is open. A and 

 B are the terminal screws of this circuit. 



Whilst it is situated equidistant from both the north 

 polarised ends, N and N 7 , of the electro-magnet, the south 

 polarised tongue c is attracted towards each of them with 

 equal force. 



When, at the sending station, the key is pressed down, the 

 current of the local battery circulates in the primary wire of 

 the induction coil. A momentary induced positive current 



