Electromotive force can be defined as the induced voltage, produced by a changing magnetic flux, whose polarity is derived by knowing the direction of the induced opposing magnetic flux .
Consider the following example. Let there be a increasing (so, changing) uniform magnetic field with time in space. Let there also exist a circular wire containing a series resistor in this field, whose area vector is parallel to the direction of the described field. By Faraday's law, an electromotive force is induced in the loop, and this voltage is developed across the series resistor. The polarity of this voltage is of special interest.
Faraday's Law (along with Lenz's law) implies that nature tends to oppose changes in equilibrium (to maintain a 'balance'). So, because there is an increasing magnetic field through the loop, Faraday's Law states that an induced magetic field is produced by the loop whose increasing direction opposes that of the original field. The direction of the induced field defines the current through the loop, and thus the polarity of the voltage (EMF) across the series resistor.
The image below (provided by Addison Wesley Longman, Inc) illustrates this. Note that the B-field in blue is increasing with time, and that the resistor is not shown here but a positive voltage will be observed across point a to point b.
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