A Hall probe measures flow in a pipe of salty water. What decides the sign of the voltage it reads?

1. A flowing fluid carries both positive and negative free charges along with it. 2. Both signs move in the same direction, since the whole fluid is moving. 3. The magnetic force pushes positive and negative carriers to opposite sides, so both contribute the same sense of voltage. 4. The sign of the reading therefore depends only on the directions of B and v. 5. This is unlike the Hall effect in a solid conductor, where opposite carrier signs give opposite Hall emf. 6. The pipe diameter sets the size of the reading through emf = Blv, not its sign. 7. Temperature does not enter the expression for the Hall emf at all. _Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.6 The Hall Effect_