You wish to design a system to communicate privately with EM radiation inside your home. The communications should not be visible, and people outside of the walls of your home should not be able to spy on them. The system should also be safe for humans. What kind of radiation would be best?

Microwaves

Explanation:

A wave is a phenomenon that causes a disturbance of the particles of a medium  without a permanent displacement of the particles. It transmit energy from one end of the material to another during propagation. It can be classified into mechanical waves and electromagnetic waves.

Mechanical waves are waves that require material medium for their propagation. Examples include: sound waves, wave in a rope, wave in a slinky etc. While electromagnetic waves are waves that do not require material medium for their propagation. They move with the same velocity. Examples are: Radiowaves, X-rays, Gamma rays etc. These waves forms a spectrum, called an EM spectrum.

For the application cited in the question, the appropriate electromagnetic wave that has the required properties is the microwaves. These are usually used in secret communications (satellite) and it is safe for humans.

the amount of current in the coil

answer: the metamorphic rocks are formed when they are exposed to high pressure and high temperatures. this happens when one of the earth's plates comes in contact with the other plate by traveling in opposite directions, straight into each other or slipping under one another. the friction that is caused by this activity results in   melting of rocks and when it cools they are known as metamorphic rocks. this process needs a controlled amount of pressure and temperature otherwise it may turn into magma.