264 THE MAGNETIC CIRCUIT 



it is convenient to use the corresponding units for reluctance 

 and permeance, to which the author has ventured to give the 

 names of rel and perm. Since one maxwell is equal to 1/10 8 of a 

 weber, one perm is equal to 1/10 8 of one henry, and one rel is 

 10 8 yrnehs. Accordingly, permeabilities and reluctivities are 

 measured in perms per centimeter cube and in rels per centimeter 

 cube respectively. 



In order not to break with the established usage, the maxwell, 

 the perm, and their multiples are employed in numerical compu- 

 tations in this book, while the weber and the henry are used in 

 the deduction of the formulae, being the natural fundamental 

 units of flux and permeance in the ampere-ohm system. It is pos- 

 sible that the constant necessity for multiplying or dividing results 

 by 10~ 8 , due to the use of the maxwell, may prove to be more and 

 more of an inconvenience in proportion as magnetic computations 

 come into common engineering practice. Then the weber, the 

 henry, and their submultiples will be found ready for use, and 

 the system of magnetic units will be completely coordinated. 



Another irregularity in the system as outlined above is caused 

 by the use of the kilogram as the unit of force, because it leads to 

 two units for energy and torque, viz., the kilogram-meter and 

 the joule; 1 kg.-meter= 9.806 joules. Force ought to be measured 

 in joules per centimeter length, to avoid the odd multiplier. Such 

 a unit is equal to about 10.2 kg., and could be properly called 

 the joulecen (=10 7 dynes). There is not much prospect in sight 

 of introducing this unit of force into practice, because the kilo- 

 gram is too well established in common use. The next best 

 thing to do is to derive formulae and perform calculations, whenever 

 convenient, in joulecens, and to convert the result into kilo- 

 grams by multiplying it by g = 9.806. This is done in some 

 places in this book. 



Thus, leaving aside all historical precedents and justifications, 

 the whole system of electric and magnetic units is reduced to 

 this simple scheme : In addition to the centimeter, the gram, the 

 second and the degree Centigrade, two other fundamental units 

 are recognized, the ohm and the ampere. All other electric and 

 magnetic units have dimensions and values which are con- 

 nected with those of the fundamental six in a simple and almost 

 self-evident manner (see the table above). 



To appreciate fully the advantages of the practical ampere- 



