324 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



circuit as it moves back and forth between stops. Figure 25 is a 

 view of such an instrument, which is called a relay. The local 

 circuit which is opened and closed by the relay contains a battery 

 which supplies the large current that is required for the operation 

 of the instrument which produces the sound signals. This 

 instrument is called a sounder. It consists of an electromagnet, 



Fig. 26. Fig. 27. 



which is wound with moderately coarse wire and which actuates 

 a massive lever and produces audible signals as it moves back 

 and forth between stops. Figure 26 shows the ordinary tele- 

 graph sounder. Figure 27 shows an ordinary telegraph key. 



29. The polarized relay. The ordinary relay which is shown 

 in Fig. 25 responds to a make-and-break key. By using the 

 proper tension on the spring which pulls the lever back (see Fig. 

 25), the lever of the ordinary relay may be made to respond to 

 increase and decrease of current, whereas a quick reversal of cur- 

 rent may not affect the instrument, inasmuch as the lever may not 

 have time to move perceptibly while the current is passing 

 through zero value. 



The polarized relay is so constructed as to respond to reversals 

 of current, but not to respond to increase and decrease of current. 

 An electromagnet NN V Fig. 2Sa, is mounted, as shown, upon 

 one pole of a U-shaped permanent magnet. A light iron lever 

 a, Fig. 286, pivoted at p, passes through a slot in the south 

 pole 5S of the permanent magnet, between the poles NA\ of 

 the electromagnet, and plays between the stops /' and p" . 

 This lever a is magnetized inasmuch as it bridges over from the 



