4 THE MAGNETIC CIRCUIT [ART. 2 



as in Fig. 1. When the current in the loop of wire circulates in 

 the direction of rotation of a right-hand screw (toward the reader 

 on the left), the lines of force within the loop point in the direc- 

 tion of the progressive movement of the screw (upward) . The 

 rule can be reversed by saying that when the direction of the 

 lines of force around a wire is that of the rotation of a right-hand 

 screw, the current in the wire flows in the direction of the pro- 

 gressive movement of the screw. The first statement is con- 

 venient in the case of a ring winding, the second in the case of a 

 long straight conductor. Both rules can be combined into one 

 by considering the exciting electric circuit and the resulting mag- 

 netic circuit as two consecutive links of a chain. When the arrow- 

 head in one of the links (no matter which) points in the direction 

 of rotation of a right-hand screw, the arrow-head in the other link, 

 as it passes through the first, must point in the direction of the 

 progressive movement of the screw. 



2. Magnetomotive Force. Experiment shows that the mag- 

 netic field within the ring (Fig. 1) does not change if the current 

 and the number of turns of the " exciting " winding vary so that 

 their product remains the same. That is to say, 500 turns of 

 wire with a current of 2 amperes flowing through each will pro- 

 duce the same field as 1000 turns with 1 ampere, or 200 turns with 



5 amperes, because the product is equal to 1000 ampere-turns in 

 all cases. Even one turn with 1000 amperes flowing through it 

 will produce the same effect, provided that the turn is made of 

 a wide sheet of metal spread over the whole surface of the ring, 

 so as to make its action uniform throughout. 



The. reason for the above can be seen by considering 1000 

 separate turns with a current of 1 ampere flowing through each 

 turn, and each turn supplied with current from an independent 

 electrical source, say a dry cell. Connecting all the cells and all 

 the turns in series gives 1000 turns with one ampere flowing 

 through each. Connecting the cells and the turns in parallel 

 results in one wide turn with 1000 amperes of current in it. 

 Such changes in the electrical connections cannot affect the action 

 of each current outside the wire, because the value of the current 

 and the position of the turn is the same in both cases. Hence, 

 the magnetic action depends only upon the number of turns each 

 carrying 1 ampere, in other words, it depends upon the num- 

 ber of ampere-turns. 



