148 MAGNETS AND ELECTRICITY 



liquid, and from copper to zinc outside of the liquid. A group of con- 

 nected cells is a battery. 



The simple cell is easily polarized by the hydrogen which collect* 

 upon the copper (or carbon) plate. Polarization is prevented, in 

 DanielFs cell, by a layer of copper sulphate solution between the zinc 

 and the copper. In other cells oxidizing materials are put into the 

 solution, or mixed with the carbon. These convert the hydrogen into 

 water. 



Wires carrying currents are magnets. Electromagnets are formed 

 when soft iron is put into coils carrying currents. 



The telegraph is essentially an electromagnet, with a key for making 

 and breaking the circuit. 



The electric bell is an electromagnet in which the armature carrying 

 the hammer continually breaks the circuit, causing the hammer to 

 vibrate back and forth. 



The resistance which the current meets in passing through con- 

 ductors appears as heat. This is the principle of electric lighting and 

 heating devices. 



Electroplating is due to the electrolysis of solutions of the com- 

 pounds of certain metals, such as copper, silver, gold, nickel, etc. 



A dynamo is a device in which coils of wire, moving rapidly through 

 the fields of powerful magnets, produce a current. 



A motor is a dynamo reversed. In it an electric current is changed 

 into mechanical energy. 



Men get electric power on an enormous scale by changing the 

 mechanical energy of falling water into electric currents. 



164. Exercises. 



1. If a piece of steel is held near a dynamo, it is pulled strongly 

 toward the dynamo. If held there for a short time, it becomes a 

 magnet. Explain both facts. 



2. Write in the Morse code: Will visit your school Tuesday, 

 April 9. Leave short spaces between letters and longer ones between 

 words. 



3. Read an account of the life of Morse, the inventor of the tele- 

 graph. 



