62 DIRECT-CURRENT DYNAMOS AND MOTORS 
this difference is not a necessity and need not be consid- 
ered in the preliminary determination. 
It is generally known whether the machine is to have two 
poles or more, but in any case it is an advantage for the 
designer to be free tv alter the form of the field magnet 
without being obliged to change the general dimensions 
of the armature, although the relative diameter would 
ordinarily be made greater for a multipolar machine. 
The last-named fact, however, need not increase the sur- 
face cf the armature required for carrying the conductors 
and getting rid of the heat generated in them. 
. This brings us to the consideration of the comparative ad-: 
vantages of bipolar and multipolar field magnets, which 
is a much-discussed question. Prior to 1890, the bipolar 
construction was generally employed for all direct-current 
machinery, but since that time the multipolar form has 
been adopted by nearly all manufacturers in this country 
and in Kurope, for sizes down to about 4 or 10 K. W., and 
even below this for low-speed machines, In England, 
however, it is still common practice to build bipolar gene- 
rators and motors as large as 100 or 200 K. W. 
52. Advantages and Disadvantages of Multipolar 
Machines.—The advantages of multipolar dynamos 
are the following: 
(1) Saving in weight of material for both armature and 
field magnet; 
(2) More compact and symmetrical form; 
(3) Better ventilation of armature and field magnets; 
(4) For armatures of the same diameter, the individual 
parts of the field frame are much smaller and more easily 
handled in the multipolar than in the bipolar type. 
The disadvantages of multipolar machines are: 
(1) Greater complication, owing to the increased number of 
parts and connections; 
(2) Strong magnetic side pull on armature, even when only 
slightly nearer one polepiece than another; much greater 
than in bipolar field; 
