DIRECT-CURRENT DYNAMCS AND MOTORS. 
3. 
4. 
The beginner, however, will generally be so wide of the 
mark that numerous recalculations are necessary to learn 
the effect of the different variable quantities upon the re- 
sult, and to arrive at a successful solution. The more 
trials that are made intelligently in this way, the more 
‘information is gained by the student. It is not wise, 
however, to carry out preliminary calculations elaborate- 
ly, because it wastes time and is discouraging if they are 
found to be incorrect.’ One should have the moral cour- 
age to change or discard drawings and results when they 
are not right. 
It is a common mistake to trust blindly to formulas which 
are merely shorthand, and, unless they are thoroughly 
understood, are very likely to give wrong results. Each 
quantity should be carefully scrutinized to make sure of 
its significance and relation to the other factors involved. 
Parts of a Dynamo or Motor.—The chief parts of 
these machines are the armature and the field magnet. 
The latter is usually stationary, and iscombined with the 
base and bearings to form what is called the frame of 
the machine. Inalternating current machines it is quite 
common t) have the field magnet revolve and the arma- 
ture stationary. This arrangement is very rarely at- 
tempted in diract-current dynamos or motors, because it 
necessitates ths rotation of the brushes and renders their 
adjustment extremely difficult. 
It is better to consider the armature first, because it is the 
most important element of the machine, and the other 
parts should be made to conform toit. As a matter of 
fact, any purt will have to be considered with reference 
to the others, but it is neces:ary to take up one at a time, 
and usually to go back and modify the first ones after 
the others have been evolved. 
Forms in which the Problem presents Itself.—In 
most cases the object of designing a dynamo or motor is. 
to produce a machine which will fulfill certain specified 
conditions. . 
