CHAPTER VI. 



ELECTRIC MOMENTUM. INDUCTANCE. 



75. The momentum of the electric current. Spark at break. 



The analogy between electric current strength and velocity, as 

 outlined in Art. 62, would lead one to expect an electric current 

 to possess momentum and kinetic energy very much as a moving 

 body possesses momentum and kinetic energy. In fact, this is 

 found to be the case. 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, and the intensity of this spark is 

 a rough indication of the amount of kinetic energy possessed by 

 the current. 



The amount of kinetic energy 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 circuit a t Fig. 95, possesses but little kinetic 



Iron 



energy ; the same current in circuit b possesses more kinetic 

 energy ; and the same current in circuit c possesses very much 

 more kinetic energy. When the circuit of an ordinary incan- 

 descent lamp is broken a very slight spark^ only is produced ; a 



141 



