212 



OUR PHYSICAL WORLD 



followed by one to the right meant a, one left and two right 

 meant w t etc. 



Morse, in the United States, devised an instrument depending 

 on the fact that, when an electric current is sent through a coil 

 wound about a core of soft iron, the latter becomes a magnet 

 but ceases to be one the minute the current stops. A sending in- 

 strument makes and breaks an electric current at the will of the 

 operator (see diagram, Fig. 85). This instrument consists of a 



FIG. 85. Diagram of an electric telegraph 



metal bar hinged at one end to a post, and at the other end held 

 directly over a second post by a spring. One wire from a 

 battery attaches to this second post; the other battery wire 

 runs to the ground. A wire attached to the bar runs through the 

 receiving instrument of the second station. When the operator 

 depresses the bar the. circuit is made ; when the pressure ceases, 

 the bar springs back and the circuit is broken. The receiving 

 instrument has a small iron bar held by a straight spring close to 

 one end of the soft iron core within the coil (Fig. 86). When the 

 current is made by the sending instrument, it passes to the coil 

 of the receiving instrument and magnetizes the core. The bar is 

 then forcibly drawn to the core, which it strikes hard enough to 



