Nick, unless I am mistaken (my university days are in a dim and distant future), the rotation of the wheel will create a small alternating voltage due to the magnetic nature of the sensor (like a dynamo).

The ammeter bit of a multimeter is in fact a very low value resistance across which a voltage is measured by the meter. Using the V=IR formula, the meter turns the voltage measurement into a current since it knows the value of the resistance.

So the resistance in the ammeter will enforce a small current to be generated, but I am not sure it would be of any relevance as it would be the only part of the circuit (probably what you meant).
The purpose of the meter's resistance in a full circuit is to introduce as little disruption as possible (hence its small value as it is wired in series), in this case, it's the only resistance, so by definition it is altering the circuit significantly.

Measuring voltage makes a lot more sense imo, which is what andy said afterwards