Answer
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Work Step by Step
First, to determine “which wires are paired with which”, use a resistance meter (an ohmmeter). We see from Figure 21-23 that the two input wires are electrically isolated from the two output wires. Pick 2 wires and test the resistance between them: a very small reading means those two wires are the ends of either the primary or the secondary coil. A large/infinite reading means that those two wires are not a pair.
To determine the ratio of turns on the two coils of a transformer without taking it apart, apply a known AC input source voltage to one set of leads. Measure the output voltage across the other two leads. The following is equation 21-6.
$$\frac{V_{secondary}}{V_{primary}}=\frac{N_{secondary}}{N_{primary}}$$
The voltage readings provide us with the ratio of the turns on the two coils.