AN ELECTRIC BELL SYSTEM 349 



like a magnet. If a bar of soft iron be slipped inside the 

 coil, it will act as before, but with far greater strength. 

 Such a device as this last, that is, a bar of soft iron having 

 a current of electricity passing through a wire surround- 

 ing it, is known as an electromagnet. The magnetic prop- 

 erty of an electromagnet ceases the instant the electric 

 current is shut off, and appears again when the current 

 is turned on. The electromagnet is the basis of many 

 household devices, some of which we are now going to 

 study. 



Experiment. To make an electromagnet. 



Materials: Dry cell. Push button. Insulated No. 20 copper wires. 

 Soft iron rod three inches long. Compass. Small iron tacks. 



Method and Result: (A] Connect one pole of the cell to one screw of the 

 push button. Connect the other wire to the other pole of cell and 

 other screw of push button. Bring a portion of the wire down over and 

 parallel to the compass needle. Press the button to cause an electric 

 current to flow through the wire. Result? The current flows through 

 the wire toward the zinc pole of the cell. Make diagram to show 

 direction of the current through the wire, and which way the north 

 end of compass needle is deflected. (B) Wind the insulated copper 

 wire in close layers around the soft iron rod, remove the rod, connect 

 the ends of the coil into the electric circuit. Hold one end of the coil 

 near the north pole of the compass needle. Press the button to pass 

 an electric current. Result? Hold the other end of the coil near 

 the north end of the compass. Result? Make a careful diagram 

 showing the windings of the coil and connections to the cell. Let 

 arrows indicate the direction of the current in the wire. Label the 

 north and south poles of the coil. Put the iron rod inside the coil and 

 test again. Result? 



Conclusion: How can you make an electromagnet? What rule can you 

 make to show the relation between the direction of the current of 

 electricity around the coil and the polarity of the coil? 



An electric bell system. The essential parts of the 

 electric bell system are the bell, connecting wire, the but- 

 ton, and a source of electric current, which usually is the 



