172 GENERAL SCIENCE 



motion may not only be used to ring bells and give signals but to open 

 and close other electrical circuits. This is seen in the telegraph instru- 

 ment known as the relay. 



Another manifestation of the relationship of electricity and magnet- 

 ism is seen in the production of electric energy in coils of many turns 

 of wire each made to rotate rapidly into and out of the field of a power- 

 ful magnet. The dynamo has become one of the most important of 

 the machines employed in the service of man, rivaling the steam engine 

 in the number and importance of its services. 



The transmission of electric signals through distances of thousands 

 of miles without wires is now an accomplished fact. The possibility 

 of doing this is apparently due to wave impulses in an ether medium. 

 The notable magnetic and electrical disturbances occurring here on 

 the earth at times of great disturbances in the sun probably have 

 reached the earth by means of an ether medium. 



Exercises 



1. Name an advantage (a) of horseshoe magnets over bar magnets; (6) of elec- 

 tromagnets (temporary) over permanent magnets. 



2. How is it that a magnet can affect objects of iron at a distance? 



3. Where the relay is used in telegraph work, and a short "local circuit" 

 and battery wholly independent of the main line is employed, in which 

 line is the key put for sending out signals to other stations? Where is 

 the sounder placed for making the signals received audible? Why so? 



