We have seen this with mercury, but apparently water also does it:
When an electrical charge travels through a magnetic field, in a direction perpendicular to the field, it is deflected in a 3rd dimension in a direction perpendicular to both the direction of its own movement and the direction of the field. The result in this experiment is that the water rotates. The same principle is used in the cathode ray tube used in old television sets. There the charges move through vacuum, whereas here they move through water.
The rule that determines which of the two perpendicular directions will be the case is usually called "Fleming's Left Hand Rule". See an explanation of this rule here (Archive.org, Archive.is). Exactly why the rule is that way, and not the other way around, requires a longer explanation, and they touch upon it on that page.
If the magnet is flipped, or if the current direction is flipped, then the rotational direction will also flip. If both magnet and current is flipped, the rotational direction will not change.
I should also clarify what I indicated in my original description that it is the ions in the water (sodium ions and chloride ions) that are deflected (causing rotation) and they in turn cause the water to rotate with them, so it is not the water directly that rotates so to say.
I got a question related to this:
Is it because of the polarising effect of water molecules?
Here is what I replied:
Not exactly, it is because ions from the salt in the water (i.e. charges) are moving between the center pin and the circumference. Charges moving in a magnetic field are deflected. The water is pulled along with them. There are both negative and positive ions, so they move both towards the pin and away from the pin. The net effect of those two radial movements is zero so to say, so water is not moving in any of these two radial directions. But both the negative and the positive ions move in the same deflected direction (which is one of the two directions that are perpendicular to the radial directions) so there is a net effect of that movement, and that pulls the water along. So rotation occurs.
Here is another related question:
Which charge is rotating ? Na +ive or Cl -ive ?
Both are rotating and both have the same rotational direction.
The rotation of a moving charge is determined by the direction of the magnetic field and the direction of the electrical current that the moving charge corresponds to. The Na+ ions and the Cl- ions are of course moving in opposite directions, but they both correspond to an electrical current going in one and the same direction.
In other words, when Na+ ions are going in one direction, they represent a current going in the same direction as the current corresponding to Cl- ions going in the opposite direction compared to the direction of the Na+ ions.
You can understand this easily if you think of a simple electrolyis situation. There you have positive ions going in one direction, and you have negative ions going in the opposite direction. But if you think of the electrical current, you have only one electrical current, and it is of course going in only one direction. So positive ions going to the left correspond to the same direction of electrical current as negative ions going to the right.