LESSONS IN PHYSICS. 95 



them together is called closing the circuit. Opening the circuit 

 and filling the gap with an instrument of any kind is called intro- 

 ducing the instrument into the circuit. 



The tumbler of acid with the two metals is called a voltaic cell, 

 the metals are called elements. There are several different kinds 

 of cells, but all are constructed on the same general principle. A 

 number of cells working together form a battery. By enlarging 

 the plates of the cells batteries of low resistance are made, but 

 they furnish great quantities of electricity. By increasing the 

 number of cells, batteries of high resistance and great intensity 

 may be made. No conductor, whether liquid or solid, is perfect ; 

 each resists the passage of electricity more or less. The resist- 

 ance in a circuit is principally in the cells, when it is called 

 internal, or in the conductors, when it is called external. 



The greatest difference between current electricity and fric- 

 tional electricity is this : Current electricity is remarkable for 

 its great quantity but feeble intensity, while frictional elec- 

 tricity is equally remarkable for its great intensity and small 

 quantity. 



If a bar of soft iron be inclosed in coils of wire, and the ends 

 of the wire be attached to the poles of a battery, the current 

 passing through the wire makes the bar of iron a magnet. Break 

 the current and the magnetism is gone. The rapidity with which 

 an iron bar may receive and part with magnetism as the current 

 is made or broken is very great; it may occur as many as 8,000 

 or 10,000 times per minute. Such magnets are called electro 

 magnets, and may be made much stronger than permanent mag- 

 nets. The induction of magnetism by current electricity is the 

 principle on which depends the working of the electric telegraph. 

 The wire connecting the battery and the coil about the soft iron 

 may be very long, even miles in length ; so the battery may be in 

 one city or town and the coil in another, and still the magnet is 

 made and an armature may be attracted every time the circuit is 

 closed. For a long time two wires were thought necessary , but it 

 was discovered that if one pole of the battery was connected with 

 the coil by a wire, and both the battery and coil connected with 



