A magnetic base is a magnetic fixture based on a magnet that can effectively be turned "on" and "off" at will; they are often used in optics

Optics

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 and metalworking

Metalworking

Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewelry. It therefore includes a correspondingly wide range of skills,...

, e.g., to hold a dial indicator

Dial indicator

Dial indicators, also known as dial gauges and probe indicators, are instruments used to accurately measure small linear distances, and are frequently used in industrial and mechanical processes...

.

The vertical post, side arm (as shown in the image at right) and the dial indicator (not shown) are connected by two swivelling connectors. These connectors allow free movement of the arms so that the indicator can be presented to the work in a suitable orientation. The magnetic base may have a "V" cut into the bottom of the base or the back, this "V" allows the base to be attached to a round bar such as the column of a drill press.

The base is made from two blocks of iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, with a round cavity bored through the centre. The halves are joined together with a non-ferrous

Ferrous

Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....

 material such as brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 or aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

. A round permanent magnet is inserted into the bored hole and a handle is attached to allow easy rotation of the magnet. This act of rotation changes the direction of the magnetic field so that it is either directed into the two halves, where the iron blocks act as keepers

Magnet keeper

A magnet keeper is ferromagnetic bar placed across the poles of a permanent magnet that helps preserve the strength of the magnet by completing the magnetic circuit, and are particularly useful for magnets which have a low magnetic coercivity, such as AlNiCo magnets.Keepers also have a useful...

 (off position), or directed so that the field traverses the non-ferrous material between the two halves (on position).
In this on position the field is effectively passing across an air gap where it can be made to do work, if this gap is bridged with another piece of iron (or steel in our case) it becomes part of the magnetic field's circuit

Magnetic circuit

A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other...

 and will be attracted with the full strength of the magnet, this is the work we want it to do—clamp

Clamp (tool)

A clamp is a fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure...

ing.


A magnetic base can therefore be attached in a variety of positions to any attractive surface, allowing the base to be positioned in the best orientation for the part to be tested. Combine this with the flexibility of movement allowed by the arms gives the operator a large range of options in positioning the dial indicator.

An alternative to the post and swiveling connectors is an arrangement whereby a series of interconnecting swivels can be set in position by tightening or loosening a central member. This member may be made from wire rope as it has to be extremely flexible yet strong in tension, the possibilities for positioning this type is probably no more than the two post type but its advantage lies in the fact that there is only one mechanism (screw) to loosen or tighten the arrangement making it easier to use, unlike the other where a third hand is often wished for.