By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
I suppose by now youâ€™ve surmised I like magnetics - the properties of magnetic materials, the measurement of magnetic fields and the means to produce those fields, via either permanent magnets or electromagnets. Thus, it is no surprise this series of articles will cover the design, construction and testing of an electromagnet. Though this particular variant is designed to rotate about an axis for specific experiments. This project is largely a consequence of recent magnetic anisotropy experiments where the need for a rotatable electromagnet was made evident by the disappointing experimental results. However, if you ever do an internet image search for â€œelectromagnetsâ€, you will see two extremes: An electric wire wrapped around a nail and connected to a battery or a large laboratory type costing thousands of dollars. Therefore, my other purpose is to show how one can build an electromagnet somewhere between these two extreme examples. This approach is not written as a â€œcookbookâ€ with specific steps you must follow. What works for me may not work for you. You will learn a little theory along the way too. I will also document my errors, leaving you the freedom to make your own mistakes. So, with these points in mind, I will show you how I designed and built an electromagnet.
Rotatable Electromagnet â€“ Initial Design Work
I began this project since my recent magnetic anisotropy experiment results showed the need for an electromagnet designed to rotate its poles around a suspended anisotropic specimen. You might remember I wanted to demonstrate the magnetocrystalline anisotropic ferrimagnetism sometimes shown by pyrrhotite â€“ an iron sulfide mineral. However, the experiment itself failed for a number of reasons â€“ one being the lack of a rotatable electromagnet. In this case, the poles oppose each other in a gap magnet configuration. They are fixed to a rotatable mount, with the rotation controlled by the experimenter. So, I had a general idea for the design. However, thatâ€™s often a long way from the final design and actual product. Therefore, my first task was reviewing texts on magnet design. I also imposed certain design constraints early. For example, I wanted to use a 6â€ diameter steel disc for the base because it was on-hand. I did not want to buy additional metal or other items if I could help it, nor were non-standard stock shapes an option. Moreover, a compact instrument would simplify its handling and storage. These three (3) constraints dictated the design to some extent, but largely avoided a sub-optimal approach.
One design reference was Electromagnets (Kroon, 1968); another was Permanent Magnet Design & Application Handbook (Moskowitz, 1976). Interestingly, my original design was essentially a copy of the design in Chikazumiâ€™s Physics of Magnetism. However, a better design from both manufacturability and efficiency standpoints was illustrated in Moskowitzâ€™s book, i.e., a Weiss-type magnet configuration. â€œManufacturabilityâ€ signified a design that would use standard stock shapes vs. a cast, horn-gap type poles. â€œEfficiencyâ€ in this context meant the design would minimize magnetic field fringing effects, thus concentrating more flux within the magnetâ€™s gap. At this point, someone might say, â€œYou are talking about both electromagnet and permanent magnet design.â€ Very true. The entity we call a â€œmagnetic fieldâ€ and its characteristics are the same in both cases. It does not matter if an electric current or aligned electron spins are producing the field. Thus, I proceeded to study three (3) design candidates with the facts and constraints above in mind.
Final Design Selection
I explored the three (3) competing designs: A shown above, with B and C in the figure below and weighed their respective pros and cons. I elected to build â€œDesign Aâ€ since it uses the magnetically efficient Weiss design and uses standard metal stock shapes. I selected the flattest 6â€ diameter by ~0.5â€ thick steel (1018) disc for the base from supplies on-hand. I then measured the discâ€™s degree of flatness with a dial indicator â€“ maximum deviation was ~0.0007â€. The photograph (next week) shows the granite surface plate, dial indicator and steel disc. I marked the steel discâ€™s high spots with indelible marker since I toyed with the idea of scraping/sanding it flat vs. facing it on the lathe. Afterwards, I began determining the magnet assembly dimensions and selecting fasteners. I will have to make my own coil form. Oh, joy. Next week, weâ€™ll continue with determine the electromagnetâ€™s initial dimensions among other tasks. See you then!
 T.E. Raney, Anisotropic Ferrimagnetism in Pyrrhotite - No Experiment is a Failure, Citizen Scientistâ€™s League, 2012.
 S. Chikazumi (S.H. Charap, trans.), Physics of Magnetism, John Wiley & Sons, Inc., New York, 1964, pg. 130.
 D.J. Kroon, Electromagnets, Boston Technical Publishers, Cambridge, MA, 1968, pp. 66-71; 109-120 and 121-129.
 L. R. Moskowitz, Permanent Magnet Design & Application Handbook, Cahners Books International, Inc., Boston, MA, 1976.