That is called an inverter.
Yes, you can use a 230 volt device on a 240 volt supply.
To convert a 240 volt power source to a 120 volt power source using a 240 volt to 120 volt adapter, simply plug the adapter into the 240 volt outlet and then plug your 120 volt device into the adapter. The adapter will step down the voltage from 240 volts to 120 volts, allowing you to safely power your device. Make sure the adapter is rated for the appropriate wattage to avoid damaging your device.
No, it will destroy the 3 volt device.
Yes
No. You will need a transformer to step up the voltage and you will possibly need to convert the plug.
There is no need to convert a 110-volt outlet to 115 volts as they are essentially the same. The slight variation is within the normal tolerance range for electrical systems. If you need to use a device that requires 115 volts, you can safely plug it into a 110-volt outlet without the need for an adapter.
No, a 240 volt device runs on 240, and a 120 volt device runs on 120. Attempting to run a device on incompatible voltage results in damage.
Use a voltage devider or a transformer.
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A device that measures volts.
It can be done with 90% efficiency.
Yes, a 110 volt device can be plugged into a 125 volt receptacle. The voltage rating on the receptacle is only there as the highest voltage supply that the manufacturer recommends their equipment be connected to.