The purpose of an adapter is to adapt the 120 volts from a wall to lower voltage that equals the batteries total voltage. This is done by two coils of wire wrapped around each other. One coil of wire has more turns than the other.

This is what the outsdie of a "typical" adapter looks like. Notice the "output" description.	This is the inside of a simple, cheap, adapter. The two coils of wire mentioned above are wrapped in blue.

Choosing the right adapter:
The voltage on the battery side of the adapter is determined by the sum of the voltages of all the batteries. For example; if a radio uses four 1.5 v batteries then the adapter would have to be rated at 6 volts; (1.5 v + 1.5 v + 1.5 v + 1.5 v = 6.0 v). If the adapter is less than 6 volts, then not enough energy will be supplied to run the radio. If the adapter is rated at more than 6 volts then electronics inside the radio can physically melt. The other variable to take into consideration when choosing an adapter is the current.

This is a question of supply and demand. The radio demands a specific amount of current. The adapter must be able to meet this demand. If the adapter cannot supply enough current then it overheats. This will cause it to do one of two things. (1) Get so hot the case melts -fire is a possibility. (2) The electronics inside the box melt. To summarize, the current rating on the adapter must meet or exceed the current needs of the device it is plugged into.

The plug and the appliance are polarized. the polarization of the device and the adapter must be identical. If they are backwards, its like putting the batteries in backwards. The device will not work.

To summarize

1. The voltage must be matched. This is like putting in too many or too few batteries.
2. The polarity must be matched. This is like putting the batteries in backwards.
3. The adapters current must meet or exceed the devices need. To little adpater power and the adapter runs hot or melts. Too much available power the adapter works fine and even operates cooler than normal.