For the second part of this lab, we were tasked with making the bottom bolts positive and ground to see how the current and heat flow. Above you can see that the heat dissipates near the bottom bolts and completely ignores the top bolt.
When the bottom bolts are positive and ground, the current flows from one bottom bolt to the next while ignoring the top bolt entirely. This shows that current flows through the least resistive path.
In the photo above, the voltage and ground were switched, this time the bottom bolts were 100mV and -100mV, and the top bolt was ground. As you can see the bottom bolts are white hot, while the ground is not as hot.
Due to the 100mV source and -100mV source being close together, the currents cancel each other out leading to the lack of current in the top bolt.