LESSONS IN PHYSICS. 99 



storage battery consists of two lead plates, which have been 

 coated with the red oxide of lead, and these with a layer of paper 

 or cloth between them suspended in dilute sulphuric acid. When 

 these plates are connected with the terminals of a dynamo, a 

 portion of the water is decomposed, the lead oxide of the posi- 

 tive pole is peroxidized, and that of the negative pole reduced 

 to metallic lead of a spongy form. The electrical energy has been 

 transformed into chemical energy. The plates will remain in 

 this condition for several days if the circuit is left open, and may 

 be transported for long distances and used as ordinary voltaic 

 cells are used. They are used for moving cars, boats, wagons, etc. 

 The principles just given enable us to explain the action of the 

 Bell Telephone. It consists of a permanent bar magnet with a 

 coil of wire about one of its poles; a thin disk of sheet iron, 

 called a diaphragm, very near but not touching the pole of the 

 magnet ; two wires, one leading from the coil to the second in- 

 strument, the other to the earth. At the other end of the line is 

 a second instrument of the same kind. The speaker puts his lips 

 to the mouth of the instrument and speaks, while the listener at 

 the other end hears what is said. The air waves of the voices 

 vibrate the diaphragm of soft iron ; these motions to and from 

 the pole of the magnet strengthen or weaken the magnetism, and 

 this sends electric pulses through the wire to the distant station ; 

 the electric pulses from the transmitting telephone act through 

 the coil of the receiving telephone at the other end of the line, 

 and alternately strengthen and weaken its magnetism, and as 

 the strength of the magnet varies the thin disk springs back and 

 forth. These vibrations of the disk produce air waves, which 

 enter the ear of the listener. Vibrations in the disk of the re- 

 ceiving instrument are the same as those of the disk in the trans- 

 mitting instrument, and the air waves formed by the disk in the 

 receiving instrument are the same as those which caused the 

 vibrations of the disk in the transmitting instrument; and the 

 two sets of air waves will affect the ear in exactly the same way, 

 so that the voice at the transmitting instrument is reproduced in 

 the receiving instrument; hence by the telephone sound waves 



