I am investigating an experiment between the current in a wire and its heating effect. I have devised a circuit where I use a variable resistor to change the current which will be measured on an ammeter. The wire will run through water in a styrofoam cup. I will leave the circuit running for a fixed period of time and measure the increase in temperature in the water. This will give me a graph with axises of temperature and current and should be a straight line. There is one problem. As the wire heats up the resistance and so the current will change, thwarting my results. Any ideas?