2 Responses

1. Let us make a brief exception to the black-box viewpoint, and consider what happens inside an electrochemical cell. Inside the cell, anions (negatively charged species) moving toward the anode make a positive contribution to the conventional current. Similarly, cations (positively charged species) moving toward the cathode make a positive contribution to the conventional current. The rule anions-to-anode, cations-to-cathode applies only inside the cell. This rule is required by the fact that current obeys a conservation law; current that flows into the cell at the anode must flow through the cell and then out the cathode. Outside the cell, current flows toward the anode; inside the cell, current flows away from the anode. (By the way, it is usually assumed that outside the cell, there are no mobile anions or cations, just electrons moving via the external circuit.)

   To make the anions-to-anode rule useful, you need to remember that anions are negatively charged. The mnenonic is something of an acronym: A N(egative) ION = ANION. Conversely the mnemonic for cations is to view the ‘t’ as a plus sign: ca+ion.

2. http://en.wikipedia.org/wiki/Cathode
   Explained with diagram.