THE CONQUEST OF NATURE 



your call-bell, for example, the wire circuit extends to 

 your door, and is there broken, shutting off the current. 



When you press the button you connect the broken 

 ends of the wire, thus closing the circuit, as the saying is, 

 and the re-established current, acting through a little 

 electromagnet, rings the bell. In another case, the wire 

 may be hundreds of miles in length, to serve the purposes 

 of the telegrapher, who transmits his message by open- 

 ing and closing the circuit, precisely as you operate your 

 door-bell. For long-distance telegraphy, of course, 

 large cells are required, and numbers of them are linked 

 together to give a cumulative effect, making a strong 

 current; but there is no new principle involved. 



The simplest study of this interesting mechanism 

 makes it clear that the cell is the apparatus primarily 

 involved in generating the electric current; yet it is 

 equally obvious that the connecting wire plays an im- 

 portant part, since, as we have seen, when the wire is 

 broken there is no current in evidence. Now, accord- 

 ing to the electron theory, as previously outlined, the 

 electric current consists of an actual flow along the wire 

 of carriers of electricity which are unable to make their 

 way except where a course is provided for them by 

 what is called a conductor. Dry air, for example, is, un- 

 der ordinary circumstances, quite impervious to them. 

 This means, then, that the electrons flow freely along 

 the wire when it is continuous, but that they are power- 

 less to proceed when the wire is cut. When you push 

 the button of your call-bell, therefore, you are virtually 

 closing the switch which enables the electrons to proceed 

 on their interrupted journey. 



