The forces of interaction between two currents are governed by the interaction of their magnetic fields; this interaction can be imagined to be as though every line of force in the magnetic field were seeking to reduce its own length, while neighbouring lines of force acting in the same direction seek to move farther away from each other. The first leads to the currents which act in the same direction being attracted to each other, and the second to the currents which act in opposite directions being repelled by one another. But if coils through which steady currents are passing are passed over a core whose magnetic inductivity is very great, and therefore all the lines of force of the magnetic field pass along the core, then the displacement of either of the coils along the core in no way alters the disposition⋅ of the lines of force of the magnetic field inside the core. Insofar as the interaction between current-carrying wires is always linked with a change in their magnetic field, while in the case of a steel core, the character of the magnetic field is set by the form of the core and does not alter when the mutual positions of the coils are altered, there can be no forces of interaction between the coils.