What is magnetism? That is an attractive question indeed. 

Magnetism Includes a magnetic field with a North and South Pole. It is found in nature but can also be produced with a coil and electricity. It has an attraction to Iron.

Here is the Merriam-Webster definition.

Here is a lot more information to digest on the subject of magnetism on the Wikipedia page

Electromagnetic chucks. Unlike in a permanent magnetic chuck, there are no actual magnets inside an electromagnetic chuck. Instead, a magnetic field is created when an electrical current is applied to a coil wrapped around a mild piece of steel inside the chuck. When there is no electrical current running through the coil, there is no magnetic field. 

A benefit of this type of workholding is that there are no moving parts inside, so there is less internal wear and tear. In addition, the magnetic field height and the holding force of the chuck can be fine-tuned by applying more or less power. A disadvantage of an electro-magnetic chuck, however, is 
that constant power is needed in order for it to hold the part, and this constant power can overheat the chuck and cause heat distortion of the parts. Also, a power failure will cause such a chuck to release a part, potentially causing an accident. 

Electro-permanent chucks. This is a mix of permanent magnetic and electromagnetic technologies. The inside of an electro-permanent chuck contains actual magnets composed primarily of alloys of aluminum, nickel and cobalt (alnico), as well as a coil that is wrapped around those magnets. A one- to two-second programmed electrical impulse sent into the coil will magnetize the alnico magnets, enabling the chuck to hold the part. With a reverse impulse, the alnico magnets become completely inactive, allowing full demagnetization of the parts being machined. 

Some electro-permanent magnetic chucks also will contain additional neodymium magnets to increase holding force. These neodymium magnets cannot be fully demagnetized, however, leaving residual magnetism in the part. These chucks are more specifically called compensated electro-permanent systems.

The primary benefit of using electro-permanent chucks in moldmaking is that they provide full access to the five sides of a mold. Because the part is magnetically clamped to the chuck, there is no need for vises that might get in the way of the tool path and require multiple workholding setups. These chucks allow for face milling, edge milling and even through-drilling in one setup. 

Additionally, electro-permanent chucks do not require a constant power source to maintain activation, so the absence of power cords allows for full range of motion during machining with a moving or swivel table. Power is only needed for the two-second impulse that either magnetizes or demagnetizes the chuck. The chuck holds the part even during a power failure, and it remains cold, so there is no heat distortion of the parts. And, like the electromagnetic chuck, there are no moving parts inside, eliminating internal wear and tear.

Naturally, with every pro comes a con. Electro-permanent chucks are more expensive than electromagnetic and permanent magnetic chucks, and their magnetic fields and power can be difficult to fine-tune. This is because the strength of electro-permanent chucks increases in irregular steps, unlike electromagnetic chucks, in which the increase in strength relates directly to how much power is running through the coils. 

Read the full article at MMT here

Residual magnetism. Also known as remanence or remanent magnetization, residual magnetism is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. For example, imagine you just finished milling a mold on a new magnetic chuck, and that mold is now ready for some final details that can only be handled by the EDM process. If you milled the mold using a magnetic chuck that did not have a full demagnetization cycle, you might be left with unwanted residual magnetism that could affect some of the EDM finish work. The magnetic field of this residual magnetism can affect the spark erosion process and cause pitting on the surface being worked. 

When speaking of a magnetic chuck, a pole pitch is the total distance between the air gap and one magnetic pole. The air gaps are most often made of resin or brass. The poles are going to be a magnetic steel that allows the flow of the magnetic field. 

For example a fine pole pitch often found on a magnetic chuck for surface grinding might be a 2mm pole pitch. This includes a 1.5mm steel pole and 0.5mm airgap made of brass.

The smaller the magnetic pole pitch is, the lower the magnetic field will travel.

The larger the pole pitch, the higher the magnetic field will be.