DIRECT-CURRENT DYNAMOS AND MOTORS. | 81 
Length in cm. = 2.54 X Length in inches, 
or L = 2.54 xX L’, no 
Lines per square inch 
Lines per square cm, = B45 > 
B’ 
or B = 6.45? 
in which the inch mark (") is used as an index to desig- 
nate the quantities expressed in English measure. 
Substituting the above values, formula (40) becomes: 
” B’” 
The permeability of air being practically 1, the ampere- 
turns required to overcome the reluctance of the air gap are: 
Bie. S018 KS Rdg gi eae be dite si (42) 
in which B,” is the flux density per square inch of gap 
area, and /,” is the length in inches of the magnetic paths 
in both air gaps. 
Since in (39)is the magnetizing force, H, in gilberts per 
7 
centimetre, it follows that .313 x “ in (41) is the ex- 
pression for the magnetizing force, in ampere-turns, re- 
quired per inch length of the circuit in order to cause. a 
flux density of B” lines per square inch. Designating 
this specific magnetizing force in English units by H’, 
we have: 
“MS Gap Speers womens tee ooeeee(43) 
In practice, magnetization curves are used in which the 
values of the specific magnetizing force H’, correspond- 
ing to various densities B’, are plotted for the different 
kinds of iron employed, so that the calculation of - the 
ampere-turns for any part of a magnet circuit resolves 
itself into the very simple operation of finding from: the 
curve the specific magnetizing force H” for the particu- 
