64 - PRINCIPLES OF ELECTRICAL DESIGN 



will be found to involve less time and labor than might be 

 supposed. The student following the courses at an engineering 

 college does not unless he has had outside experience ap- 

 preciate the value of his time. Time may be used, abused, or 

 wasted; and when a concrete and definite piece of work has to 

 be done, the time spent upon it, not only by the workman, but 

 also by the designer, may be of no less, or even of greater, im- 

 portance than the cost of the materials. The case in point 

 exemplifies this. If the required magnets are small, and but 

 two or three are likely to be wanted, the designer should not 

 spend much time on refinements of calculation and in endeavor- 

 ing to reduce the cost of manufacture to the lowest limit; but if 

 the magnets are of large size and several hundred will be re- 

 quired, then time spent by the designer in comparing alternative 

 designs and in striving to reduce material and labor cost, would 

 be amply justified. These considerations and conclusions may, 

 to many, appear elementary and obvious; but they emphasize 

 the importance of what is generally understood by " engineering 

 judgment" which is rarely acquired or rightly valued until after 

 the student has left school. 



Before taking up the design of another form of magnet, it 

 may be well to state that the method of procedure here followed 

 in the case of a horseshoe magnet is not put forward as being 

 necessarily the best, or such as would generally be adopted by 

 an experienced designer. It serves to illustrate much that has 

 gone before, and emphasizes the fact that, even if the designer 

 must make some assumptions and do a certain amount of guess- 

 work at the beginning, and during the course, of his design, he 

 can always check his results when the work is completed, and 

 satisfy himself that his design complies with all the terms of the 

 specification. 



17. Circular Lifting Magnet. The electromagnet of which 

 Fig. 22 is a sectional view is circular in form. Its function is to 

 lift a ball of steel weighing, say, 4,000 lb., which, on the opening 

 of the electric circuit, will fall upon a heap of scrap iron. This 

 device is referred to colloquially as a "skull cracker." The 

 diameter of a solid steel sphere weighing 4,000 lb. is approxi- 

 mately 30 in. If the outer cylindrical shell forming one of the 

 poles of the magnet has an average diameter of 21 in., it will 

 include an angle of 90 degrees, as indicated in Fig. 22, and lead 

 to a design of reasonable dimensions. If the required width of 



