Diode IV experiment Ive been looking at experiments to look at the IV relationship for a LED, but I see most of them use a rheostat instead of a series variable resistor and my question is, what is the advantage of using a rheostat over a variable resistor in this case?

 A: There are devices which sometimes look like this and they are often called rheostats.

When terminals $C$ and $A$ or $B$ are being used it is probably being used as a variable resistor.
When all three terminals are being used is is probably being used as a potential divider / potentiometer.
So there are two possible arrangements which can be used to vary the current thorough and the voltage across a device.as shown below.

The left hand diagram is its use as a variable resistor and the current in the circuit is $I= \dfrac{\mathcal E}{R_{\rm D}+r}$.
This arrangement is not much use if $R_{\rm D} \gg r$ as changing $r$ does not change the current by very much.
The right-hand diagram varies the voltage across the device and works fine as long as $r \ll R_D$.
If this condition is not satisfied then the standard voltage divider equation $V_{\rm out} = \dfrac{r}{R+r}\mathcal E$ ceases to apply because the $r$ is in parallel with a much smaller resistor $R_{\rm D}$ and it is $R_{\rm D}$ which controls the output voltage.
The measurement of the $IV$ characteristics of a diode present a number of problems in that a diode behaves like a large value resistor when it is reverse biassed and  also below the knee voltage when it is forward biassed.
So in these regions the potential divider circuit is to be favoured.  
However above the knee voltage the resistance of forward biassed diode is low and then the variable resistor arrangement is to be favoured to have good control of the currents flowing through the diode.
On top of this complication there the important consideration of the arrangement of the voltmeter and the ammeter.
If the diode resistance is low then the arrangement shown in the OP's diagram is a good one in that the voltmeter measures and voltage across the diode whilst the ammeter measures the current through the diode and the voltmeter which is effective the current through the diode because the resistance of the voltmeter is so much higher than that of the diode.  
However when trying to measure the IV characteristic when the resistance of the diode is high the ammeter should be put in series with the diode to measure the current through the diode and the voltmeter should be placed across the diode and the ammeter to measure the voltage across the diode (and the very small voltage across the ammeter).
For "intermediate" values of diode resistance corrections might have to be made for the voltage drop across the ammeter and/or the current passing through the voltmeter.
A: As has been pointed out in the comments, a rheostat is a two-terminal adjustable resistor.

There is no rheostat in the circuit as drawn.
There is however, a potentiometer which is a three-terminal variable voltage divider (it is possible to connect a potentiometer to use as a rheostat but not the other way around).

The advantage that I see to the use of the potentiometer as drawn is that it is possible to adjust the voltage across to diode from zero volts (shuttle all the way to the left) up to the battery voltage (shuttle all the way to the right).
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