APPENDIX A. 

 INDUCTANCE AND CAPACITY. 



1. A spark at break. When an electric circuit is broken, the 

 current continues to flow across the break for a short time, pro- 

 ducing an electric arc or spark. This action of the electric cur- 

 rent is suggestive of momentum, and, consequently, we shall 

 speak of the momentum of the electric current. The intensity of 

 the spark is a rough indication of the amount of this momentum. 



The amount of momentum associated with a given current in 

 a circuit made of a given length and size of wire depends upon 

 the shape of the circuit and upon the presence of iron near the 

 circuit. Thus, a current in a circuit a, Fig. i, has but little 



iron 



Fig. 1. 



momentum ; the same current in circuit b has greater momen- 

 tum ; and the same current in circuit c has very much greater 

 momentum. Thus, when the circuit of an ordinary incandescent 

 lamp is broken, a very slight spark is produced ; the same 

 amount of current flowing through a coil of wire produces a 

 much more intense spark when the circuit is broken ; and a spark 

 several inches in length may be produced if the coil of wire sur- 

 rounds a core consisting of a bundle of iron wires. 



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