Chap, iv.j EXTRA CURRENT. 45 



of the current, that is, in the reverse direction to the 

 battery current. 



Thus there are two extra currents, one appearing 

 on the closing of the circuit, tending to oppose the 

 establishment of the current, that is, in the opposite 

 direction to, and therefore weakening, the battery 

 current. This is the INVERSE EXTRA CURRENT, or 

 extra current of closure. The second appears on 

 interrupting or opening the circuit, tends to oppose 

 the interruption, and is, therefore, in the same direc- 

 tion as the battery current, and for the instant inten- 

 sifies it. This is the DIRECT EXTRA CURRENT, or extra 

 current of opening. It explains why the spark 

 obtained on opening is more intense than that on 

 closing. 



Now, in an ordinary induction coil, the production 

 of these extra currents in the coil of the primary 

 circuit occasions corresponding differences in the 

 induced currents of the secondary coil. For by the 

 inverse extra current the primary current is at the 

 moment of closing weakened for an instant, and, on 

 this account, attains its maximum strength only 

 gradually. On the other hand, the direct extra 

 current is suppressed on opening, because no arrange- 

 ment is made to maintain the circuit of the primary 

 coil when that of the battery is opened, and it also is 

 broken. Thus, the" current of the primary coil is 

 suddenly, not gradually, interrupted. On closing, 

 therefore, the primary current gradually attains its 

 maximum ; on opening, it is suddenly interrupted 

 when at its full strength. This must affect the cur- 

 rents of the secondary coil, which are induced by the 

 opening and closing of the primary current. Fig. 25 

 graphically represents the differences. The part of the 

 figure enclosed by bracket I applies to the primary 

 current ; that enclosed by bracket II applied to the 

 induced currents of the secondary coil. AA is the 



