CHAP. I] I LUX AND MAGNETOMOTIVE FORCE 7 



are used, namely the kilo-maxwell, equal to one thousand max- 

 wells, and the mega-ma \\vell, equal to one million maxwells. 

 These two units are sometimes called the kilo-line and the mega- 

 line, the word " line " being used for the word maxwell, as 

 plained above. 1 



Prob. 1. The flux within the coil (Fig. 1) is equal to 63 kilo-maxwells. 



A test coil of five turns is wound on the exciting coil so as to be linked 



with the total flux. What voltage is induced in this test coil when the 



current in the main (exciting) coil is reduced to zero at a uniform rate 



wn B6COI Ans. O.4.") millivolt. 



Prob. 2. At what rate must the flux be varied in tin- preceding 

 problem in order to induce one volt in the test coil ? 

 'A-^. S.'.T'Vcc.i Ans. O.L J wehcr (LM) megalines) p(>r second. 



Prob. 3. When the flux varies at a non-uniform rate show that the 



voltage induced in the test coil at any instant is equal to (d0/dt) X1Q-, 



where t is time in seconds, and is the flux in maxwells. Show that 



onent of 10 must be -2 instead of -8 if the flux is expressed in 



megalines. 



4. The Reluctance of a Magnetic Path. Experiment shows that 

 the total flux within the coil ( Fig. 1) is proportional to the applied 

 magnetomotive force, when the space inside is filled with air. 

 Therefore, a relation similar to Ohm's law holds, namely, 



........ (1) 



w here M is the magnetomotive force in ampere-turns, is the flux- 

 in maxwells, and (R is the coefficient of proportionality bet\v 

 the two, called the reluctance of the magnetic circuit. Script (R is 

 used to distinguish reluctance from electric resistance. The mag- 

 netomotive force M is the cause of the flux; or, with reference to 

 an electric circuit. M is analogous to the applied electrons 

 force. is analogous to the resulting current, and the reluctjt 

 (R takes place <>i the electric resistance. Therefore, eq. (1) is 

 known as Ohm's law for the magnetic circuit. Of course, the 



lily of rrrntiiiK new in <iie is a great 



advantage of the metric or decimal systeti New units are gener- 



ally understood, by the use of Latin an ihrir 



numerical relation to the fundamental unit I <>r irM.mre. it ia perfectly 



lie to uae such unita M dcci-ampcre and hecto-volt, in spite of the 



t that they arc not in general uae. Anyone familiar \\ith the agreed 



fixes will know that t!,.- unita spoken of are equal to one-tenth of one 



ampere, and to one hundred volt*. See Appendix I on the Ampere-Ohm 



>y-t.-n. 



