CHAP. I] FLUX AND MAGNETOMOTIVE FORCE 3 



of the field; this fact can be demonstrated by a small compass 

 needle. The positive direction of the lines of force is indicated 

 in Fig. 1 by arrow heads. The direction of the current is shown 

 in the conventional way by dots and crosses; namely, a dot 

 indicates that the current is approaching the observer, while u 

 cross indicates that the current is receding. 



The magnetic state within the coil can also be explored by a 

 small test-coil inserted into the field and connected to a galvanom- 

 eter. When this coil is properly placed with respect to the field 

 and then turned about its axis by some angle, the galvanometer 

 shows a deflection, because a current is induced in the coil by the 

 magnetic field. There are also other means for detecting a mag- 

 netic field, for which the reader is referred to books on physics. 



The total magnetic field produced by a current is called a 

 magnetic circuit, by analogy with the electric and the electrostatic 

 circuits. Experiment shows that the magnetic lines of force are 

 always closed curves like the stream lines of an electric current, or 

 like the lines of electrostatic displacement (when these are com- 

 pleted through the conductors). 



Fig. 1 exemplifies a fundamental law of electromagnet ism; 

 namely, an electric current creates a magnetic field in such direc- 

 tions that the lines of force are linked with the lines of flow of the 

 current, in the same manner that the consecutive links of a chain 

 are linked together. This law admits of no theoretical proof, and 

 must be accepted as a fundamental experimental fact. Wherever 

 there is an electric circuit there is also a magnetic field linking 

 with it. The two are inseparable, and increase and decrease 

 together. Each form of an electric circuit with a certain stn 

 of current in it corresponds to a definite form of magnetic field. 

 It is possible that the electric current and the magnetic field 

 ut two different ways of looking upon one and the same 

 phenomenon. 



The linkages of magnetic lines with a current are acen more 

 clearly in I-'i.ir. 11. which shows the magnetic field produced by a 

 loop of wire. aa. It will be seen that the arrangement in I 

 is more suitable for an elementary study, because the field is much 

 more uniform, especially if the radial thickness of the ring in 

 small as compared to its mean diameter, so that all the linea of 

 practically the same length. 



The same right-hand screw rule applies in the case of Fir. 1 1 



