An electrodynamic ammeter uses an electromagnet instead of the permanent magnet of the d'Arsonval movement. This is called a ferromagnetic-core or iron-core electromagnet. The "electromagnet" (or ""coil"") provides the driving force to close the contacts. The primary winding is located on the upper electromagnet. A "tractive electromagnet" applies a force and moves something. The shutter is operated by an electromagnet. The change in weight when the electromagnet is turned on is proportional to the susceptibility. Witnessing various experiments with Jackson's electromagnet, Morse developed the concept of a single-wire telegraph. For Daedalus, this process was run within a large electromagnet which formed the rocket engine. MO discs were written using a laser and an electromagnet. It then moves at a constant speed over an electromagnet. Cycling is achieved by interrupting the current through the electromagnet. These two impurities are thus removed by running an electromagnet over the mixture. The use of an electromagnet to find subsurface indications is difficult. William Sturgeon invented the electromagnet in 1824.