Conventions for writing down the electrochemical cell

however, the Pt(s)IH2(g)IHCl(aq) electrode is linked to a Mg(s)IMg (aq) electrode, deflection occurs in the opposite direction indicating that in this cell the potential difference set up when the circuit is completed causes the electrons to flow from the Mg(s)IMg (aq) electrode to the Pt(s)IH2(g)IHCl(aq) electrode. Reduction is now occurring at the Pt(s)IH2(g)l HCl(aq) electrode, i.e. H30+(aq) + e /2H2(g) + H20(l), making it the positive electrode. Oxidation occurs at the Mg(s)IMg (aq) electrode, i.e. Mg(s) — Mg + (aq) + 2e, making this the negative electrode. 

Experiment can thus determine the direction of flow of electrons in the external circuit and can determine which is the positive electrode and which is the negative electrode of the cell set up. It now remains to decide on a standardised manner of writing down the cell which would allow inferences to be made about the polarity of the electrodes, reaction at the two individual electrodes and the direction of flow of the electrons in the external circuit. 

The cell is written down from left to right so that the inference is that  

It now remains to find out experimentally whether the reactions which actually occur are 

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