168 LABORATORY LESSONS IN GENERAL SCIENCE 



as possible let the connection with the cell be broken, and 

 note the effect. Repeat several times to be sure of the con- 

 dition of the coil as the circuit is (a) opened; (b) closed. 

 State the results. 



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6. Observe closely the results when the teacher connects 

 a dry cell to an electric bell in working order. (Use two cells 



if necessary.) 



Describe the course 

 of the current through 

 the bell, tracing it very 

 carefully with such as- 

 sistance as may be 

 needed. 



Notice the effect on 

 the hammer of the bell 

 as the circuit is closed 

 and opened. Explain 

 (a) why the hammer 

 is drawn towards the 



FIG. 74. The 



electric bell, and how it 

 operates. 



coil; (b) why it flies 

 back from the coil. 

 Explain how the "make and break" of the circuit .is ac- 

 complished automatically with the battery connections 

 continuously maintained. 



State several possible causes why electric doorbells may 

 fail to operate. What may be done to locate the particular 

 cause of trouble in order to remedy it ? 



7. Connect a telegraph key into the circuit between the 

 battery and the bell; make use of it to close and to open 

 circuit, thus ringing the bell at pleasure. Describe the 



