234 THE MAGNETIC CIRCUIT [ART. 68 



In the position of the positive brushes just preceding that 

 marked 1, the coil CD is not short-circuited, and carries the full 

 current /i, being the first coil of the right-hand branch. The 

 lead d is idle, and the total current 27 1 is delivered to the brushes 

 through the leads c, m, and n. In the position of the positive 

 brushes just after that marked 2 n the coil CD is again not short- 

 circuited, but is carrying the full current I\ in the opposite direc- 

 tion, being the first coil of the left-hand branch. 



In the positions of the brushes between 1 and 2 the coil CD 

 is short-circuited by the brushes through the leads c and d, 

 and the current in the coil changes gradually from +/i to 1\. 

 If the coil possessed no inductance, the variation in the current 

 would be practically determined by the contact resistance 

 between the brush and the commutator, the resistance of the 

 coil itself and of the leads being negligible (with carbon brushes). 

 Under these conditions the current in the short-circuited coil 

 would vary with the time according to the straight-line law, 

 and the current density under the heels and the toes of the 

 brushes would be the same. This is called the " pure resistance " 

 commutation, or the perfect commutaton, because it is not accom- 

 panied by sparking. Such a commutation is approached in 

 machines with interpoles, when the effect of the inductance is 

 correctly compensated for by the commutating flux (Art. 54). 



In reality, the short-circuited coil possesses a considerable 

 inductance, which has the effect of electromagnetic inertia, 

 retarding the reversal of the current. Consequently, at the 

 beginning of the commutation period the lead d and the corre- 

 sponding commutator segment do not carry their proper share 

 of the current, which they would carry with a perfect com- 

 mutation. At the end of the commutation period the current 

 must then be reversed quickly, because the whole current must 

 be transferred from the lead c to the other leads. If the inductance 

 is considerable, the current in the lead c is still of a considerable 

 magnitude when the toe of the brush is about to leave the 

 corresponding commutator segment. Therefore, the last period 

 of the reversal is accomplished through the air between the 

 brush and the segment, in the form of an electric arc. This 

 is known as the sparking at the brushes. Besides, during the 

 last moments of reversal, the current density under the toe 

 is much higher than the average density under the brush, and 



