 Let's see what happens when we make a battery here on this piece of paper. So we have a copper and we're adding a copper nitrate solution around it and we have zinc and the zinc nitrate solution we're going to add around that and in the middle we're going to put potassium nitrate. So now we have a potential difference between the two metals. If we put our voltmeter probes on each metal you see we get a voltage. Here we have the red lead on copper and the black lead on zinc. If we reverse them and put the red lead on zinc and the black on copper we get a negative number. What we want in a spontaneous reaction is a positive value. So if we reverse the probes we get a positive value with copper on the red lead. How I remember what's been oxidized and what's being reduced from an experimental standpoint is reduce red cats. To me that means reduction takes place at the cathode and that's the red lead on our voltmeter in this battery. So copper is being reduced.