96 PRACTICAL LESSONS IN SCIENCE. 



the earth, that the earth would complete the circuit. As the re- 

 sistance of the earth is practically nothing, less powerful batteries 

 were needed, as they had only to overcome the resistance of one 

 wire instead of two. The distance may be so great that the bat- 

 tery at one end cannot entirely overcome the resistance, in which 

 case relay batteries are introduced. In addition to the battery, 

 coil and line wire, an instrument called a key for opening and 

 closing the circuit is necessary, and a mechanical device for the 

 purpose of registering the action of the armature, which may be 

 by blows on a sounder or by marks on paper moved by clock 

 work. If the circuit is closed for an instant a dot is made on the 

 paper, or a short sound on the sounder; if for a longer time, a 

 line is made on the paper or a longer sound, and the differences 

 between the blows on a sounder, made by the dot or line closure 

 is easily distinguished by the ear, so that all first-class operators 

 work by sound. The telegraphic alphabet, consisting of dots 

 and dashes, is called the Morse alphabet, and the telegraph is 

 usually known as the Morse telegraph. But there is reason to 

 believe that while Morse introduced them to the world, they were 

 really invented by another man of whom the world has seldom 

 heard. Electric bells and electric clocks are based on the same 

 principle as the telegraph. 



The magnetic needle whose deflection indicates electricity in the 

 connecting wires of the voltaic cell, is in fact a galvanoscope. 

 It may be made so that the needle hangs over a graduated circle 

 under which a coil of wire is placed, and so arranged that the 

 instrument may be introduced into a circuit. 



Make two coils of wire, one containing a few feet of stout copper 

 wire, called the primary coil; and another containing a much 

 greater length of fine copper wire, called the secondary coil, and 

 make them so that the primary coil may be placed inside the 

 secondary, or removed at pleasure; connect the primary coil with 

 the battery and the secondary with the galvanometer. Now, 

 lower the primary coil into the secondary coil and the galvano- 

 meter shows the presence of a current in the secondary coil, but 

 the needle of the galvanometer soon settles to zero, showing that 



