When a current is passed through the coil the iron tends to move nearer to the coil of the wire where the field is stronger and so displaces the index needle over the scale.
When a current is passed through the wire forming the coil, the fragment of iron is drawn more into the aperture of the coil where the field is stronger and so displaces an index needle over a scale.
When a current is passed through the coil it rotates in the field and displaces the index over the scale against the control of a spiral spring like the hairspring of a watch.
If the conductor consists of a coil of wire the ends of which are connected with a suitable galvanometer, the integral electromotive force due to a sudden increase or decrease of the induction through the coil displaces in the circuit a quantity of electricity Q=SBns R, where SB is the increment or decrement of induction per square centimetre, s is the area of the coil, n the number of turns of wire, and R the resistance of the circuit.
The acid is then slowly run out by an opening in the bottom of the pan in which the operation is conducted, and water distributed carefully over its surface displaces it in the interstices of the cotton, which is finally subjected to a course of boiling and washing with water.