DIRECT-CURRENT DYNAMOS AND MOTORS. 97 
the average temperature increase with a power dissipa- 
tion of 1 watt per square inch of exposed coil surface 
being about 75° C. . 
Inserting the value of w,, given by (50) into the above 
equation for Cs, the following formula for the shunt 
current is obtained: 
in which ¢ = prescribed temperature increase of magnets, 
in degrees C., being usually limited to 5v°; 
Sy = cooling surface of magnets, in square inches. 
The values of /, and Sy, in formulas (49) and (51) respec- 
tively, depend upon the height of the magnet winding, 
which at this stage of the calculation is not yet known. ' 
We must, therefore, temporarily make an:assumption of 
its magnitude. For this purpose the average practical | 
height of the winding on magnets of different sizes is given 
in Table 2%, page 98. The winding height of a cylindrz- 
cal magnet is obtained by taking the figure given under the 
heading of ‘‘ Circular Cores” for the nearest diameter 
given in the first column. In case of a rectangular or 
oval magnet, the figure corresponding to the nearest cross- 
section given in the secondcolumn istaken. Thus, to find 
the height of the winding space of a rectangular magnet, 
4 by 6 inches, the nearest area given in the second column 
is 28.3 square inches; hence, the required winding height — 
is 2 inches in case of a bipolar machine, and 2? inches in | 
- case of a multipolar machine. | | | 
The two columns added at the right give the ratios of the 
length of the mean turn to that of the diameter, and, of ' 
course, only apply to circular magnets. These ratios are: 
tabulated in preference to the actual lengths of the 
average turn, in order to enable the ready application of 
the table to diameters intermediate to those given. The 
approximate length of the mean turn for a cylindrical 
magnet of any diameter between 1 and 36 inches can be 
directly taken from the table by multiplying the diameter 
of the core under consideration by the ratio given for the 
