Two copper wires, W and Z, are joined as shown. Wire W has twice the diameter of wire Z.

W and Z are connected in series with a cell and resistor, as shown.

State the purpose of the resistor in this circuit.
The current I in a conductor is given by the formula
            
Complete the following table, which shows ratios of quantities for wires W and Z. The first row has been completed for you.
| Ratio | Value | Reason | 
| 1 | Both wires are made of the same material | |
| 
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 | |
| 
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